Because of the geography, history, and sensitivities of the Nagorno-Karabakh conflict, accusations, allegations, and statements have been made of involvement by third-party and international actors during the Second Nagorno-Karabakh War, including in media reports. Azerbaijan has been accused of employing Syrian mercenaries during the war, including reports by the Syrian Observatory for Human Rights (SOHR). There have also been allegations of Kurdish militia from Syria and Iraq fighting on the Armenian side, and although some third-party sources had confirmed it, some publications had considered these claims "dubious". During the war, ethnic Armenian volunteers from the Middle East, Europe, and Latin America fought on Armenian side. Both sides have denied employing mercenaries in the war, but the OHCHR had stated that there were reports about Syrian fighters motivated primarily by private gain fighting on Azerbaijan's side recruited with Turkey's assistance and foreign nationals fighting on Armenian side with motivation being investigated, calling for withdrawal of any mercenaries and related actors from Nagorno-Karabakh.

Afghan militants On 7 October 2020, Artsakh's Presidential Spokesman Vahram Poghosyan said that according to intelligence, Turkish President Recep Tayyip Erdogan had reached an agreement with the leader of the Islamic Party of Afghanistan (Hezb-e-Islam) Gulbuddin Hekmatyar to involve "new terrorist groups in the war against Artsakh". On 17 October 2020, Armenia's National Security Service (NSS) stated that the Azerbaijani side is engaged in the transportation of a large amount of ammunition and mercenaries to its territory, citing the flights of Silk Way's Boeing 747 and Il-76 planes from Baku to air bases in different countries as evidence. According to the Armenian NSS, the Baku-Bagram flight was registered on 16 October, and the Bagram-Lashkar Gah-Baku flight on 17 October. The Armenian NSS reported that the same route was scheduled for 18 October and flights from Baku to Kandahar. It also notes that information on some of these flights is absent in the system of international timetables, and a number of airports do not have an international classification. On 2 October 2020, Afghanistan's declared diplomatic support for Azerbaijan prompted Armenia to push for an end to Kabul's observer status in the Collective Security Treaty Organization (CSTO), a Russian-led regional alliance.

Armenian diaspora volunteers

On 28 September 2020, the Azerbaijani MoD alleged that among the Armenian casualties were "mercenaries" of Armenian origin from Syria and a variety of Middle East countries. On the same day, the Turkish Minister of Defence stated that Armenia must "send back the mercenaries and terrorists it brought from abroad". Two days later, Azerbaijani authorities asked the international community to "adequately respond to the use of terrorist forces by Armenia". On 30 September 2020, the SOHR also stated that Armenian-born Syrian fighters were being transported to Armenia to fight against Azerbaijan. The next day, Azerbaijani authorities stated that Armenia had widely employed foreign "terrorist forces" and "mercenaries" against it, with there being evidence of people of Armenian origin from the Middle East, especially Syria and Lebanon, and subsequently Russia, Georgia, Greece, the United Arab Emirates, and other countries. A Novaya Gazeta report, citing Lebanese Ministry of Internal Affairs intelligence, stated around 500 ethnic Armenian Lebanese had travelled to fight in Nagorno-Karabakh. In early October, Komsomolskaya Pravda reported that Armenians from Lebanon, who were members of Armenian Secret Army for the Liberation of Armenia, a militant group listed as a terrorist organisation by the United States, had supported Armenia in the war. Greek City Times have reported that about 500–800 Armenians from Greece, as well as some Greeks volunteered to the war on Armenia's side. An Armenian defence ministry official had said many from the diaspora applied to volunteer without giving a precise number. Hundreds from as far afield as Argentina and the United States have rushed back to Armenia for combat training, a local instructor says. Karapet Aghajanyan, the combat instructor in Yerevan, who trains local and foreign volunteers, told Reuters around 10 Lebanese-Armenians had received training in his camp. France 24 spoke to soldiers including Armenian diaspora from France fighting in the Nagorno-Karabakh.

Israel

On 24 April 2021, the Chief of Air Defense Forces of the Armenian Army Armen Vardanyan stated that Israel has not only supplied military equipment to Azerbaijan during the Second Karabakh War, but also operated them. According to the Major General, Israel provided direct assistance through their operators and personnel during hostilities.

Kurdish militias

On 30 September 2020, Turkish sources alleged that approximately 300 Kurdistan Workers' Party (PKK) militants were transported to Nagorno-Karabakh via Iran. However, according to Eurasianet and The Jerusalem Post, these claims were not substantiated by evidence. On 6 October 2020, the Azerbaijani State Security Service (SSS) claimed, citing intercepts, which it released as an evidence, that Armenia had employed foreign mercenaries, including members of Kurdish militant groups whom Armenia had brought from Iraq and Syria, to fight Azerbaijan. On the same day, Russia's Foreign Intelligence Service alleged that there were Kurdish extremist groups, which, according to Al-Monitor, are fighting on the Armenian side, while there is no direct evidence of it. In early October, Komsomolskaya Pravda reported that there was evidence of Armenia being supported by the Martyr Nubar Ozanyan Brigade militants, part of the Syrian Democratic Forces, and associated with the PKK. However, the Martyr Nubar Ozanyan Brigade strongly denied claims of them travelling to Armenia. The commander of the group mocked the Turkish accusation, saying "We know that this perception is completely false. Besides, everybody knows that the PKK members are not in Armenia. Because it's not Kurdistan."

Pakistan

On 17 October 2020, the Armenian National Security Service stated that Azerbaijan was smuggling in a significant amount of ammunition, mercenaries and "terrorists" from Afghanistan and Pakistan. Pakistan rejected this allegation as "irresponsible propaganda", and said the claims by the Armenian government had no substance. Armenian Prime Minister Nikol Pashinyan said in an interview with the Indian news channel WION that there was "information that militants from Pakistan are involved in the war against Karabakh." He told Russian reporters about Pakistan's active role in the conflict. The President of Azerbaijan, Ilham Aliyev, stated that Pakistan had supported Baku diplomatically "but military assistance is out of the question." On 28 October 2020, based on informed sources, Kommersant reported that at least one batch of militants with weapons and ammunition were formed and sent from Pakistan's Peshawar to Karabakh. Later there were new reports about provided support by Pakistan to Azerbaijan during the war. In September 2021, the Azerbaijani MoD expressed his gratitude to Pakistan for its support during the war.

Russia

On 9 November, the day when the ceasefire agreement was signed, the Azerbaijani forces in Nakhchivan Autonomous Republic accidentally shot down a Russian Mil Mi-24 attack helicopter near Yeraskh, in Armenia. According to Anton Troianovski and Carlotta Gall of The New York Times, this potentially gave Russia a reason to intervene in the war, and the Russian president Vladimir Putin delivered an ultimatum to the Azerbaijani president Ilham Aliyev. According to Troianovski and Gall, in this ultimatum, Russia stated that if Azerbaijan did not cease its operations after seizing control of Shusha, it will intervene. The same night, an unknown missile hit an open area in Khyrdalan, near Baku, without causing any injuries, according to the Azerbaijani sources. Also, yet again on the same day, a video emerged on the social media apparently showing the Armenian forces launching a Russian-made Iskander missile into Azerbaijan. The former Head of the Military Control Service of the Armenian MoD Movses Hakobyan, after resigning from his post on 19 November 2020, stated that Armenia used an Iskander missile on Azerbaijan, though he did not say where the missile hit. Armenia's Prosecutor General's office said in a Facebook post that it would investigate Hakobyan's allegations. A Defense Ministry spokeswoman didn't respond to phone calls seeking comment. According to Can Kasapoğlu, the Director of Security and Defense Studies Program at the Centre for Economics and Foreign Policy Studies, an Istanbul-based independent think-tank, Armenia could've used Iskander missiles only with the Russian consent. Azerbaijan and Russia denied that Armenia fired an Iskander during the war,Алиев заявил, что в Карабахе не применяли "Искандеры" however, according to the Middle East Eye, Armenia had fired at least one Russian-made Iskander ballistic missile at Azerbaijan's capital, Baku in November 2020 and they were shot down by a missile defence system operated by the Azerbaijani military. In March 2021, Azerbaijan National Agency for Mine Action (ANAMA) stated that Armenia had used Iskander missiles during the battle, adding that they found the remains of two exploded Iskander missiles during the demining of the area in Shusha; this was denied by Russia.

Wagner Group On 28 September, Russian media reported that Russian private military companies were ready to fight against Azerbaijan in Nagorno-Karabakh. On 1 October, Radio Free Europe/Radio Liberty, citing a Wagner Group source, claimed they were already in Nagorno-Karabakh and participating in hostilities. The Russian military analyst Pavel Felgenhauer also stated that Wagner contractors were sent to support the Armenian forces as ATGM operators. After the war, in December 2020, a photo of a Wagner mercenary, apparently taken in front a church in Shusha during the war, appeared on the internet. Also, the Russian media leaked a message, apparently describing how the Armenian government refused to pay the Russian mercenaries for their work, and how, because of that, some of the Wagner mercenaries intended to return to Russia or defect to the Azerbaijani side. The Russian media reported that, in November, there were about 500 Russian mercenaries fighting on the Armenian side, and some 300 Russian mercenaries had taken part in the Battle of Shusha, with Victor Zlobov, a retired captain of the Russian Armed Forces, stating that Shusha was "defended mainly thanks to the Russian volunteers".

The Russian businessman Yevgeny Prigozhin, who founded Wagner Group in 2014, denied any involvement of the Russian PMCs in the war. According to the Armenian journalist Karine Ghazaryan, writing for Bellingcat, there was not "any firm evidence showing their arrival or involvement in the war". She stated that Reverse Side of the Medal (RSOTM), a media channel linked to Wagner Group, which, according to Ghazaryan, was the main source of the reports, was not the "breaking news source."

Turkey and Syrian National Army