The combatants of the war in Donbas included foreign and domestic forces.
As of February 2018, the number of southeastern army separatist forces is estimated at 31,000 of which 80% (25,000) are Donbas residents, 15% (≈5,000) are military contractors from Russia and other countries. other countries and 3% (900–1,000) are regular Russian Armed Forces personnel. This ratio has changed significantly from previous years, with "the Russian command gradually filling the armies of the 'republics' with locals". since peaking in 2015.
Separatist insurgents
Donbas People's Militia
Igor Girkin, a Russian citizen and FSB officer from Moscow who commanded the Donbas People's Militia in Sloviansk, initially denied Russian involvement in the insurgency. He said his unit was formed during the Crimean crisis, and that two-thirds of its members were Ukrainian citizens. Girkin also said that the Sloviansk insurgents had agreed to work with the leadership of the Donetsk People's Republic, despite some conflict between insurgent groups. According to a spokesman for the Donetsk People's Republic, the militants that occupied Sloviansk were "an independent group...supporting the Donetsk protest", while insurgents in Sloviansk and Kramatorsk identified themselves as members of Pavel Gubarev's Donbas People's Militia. The group's forces at Sloviansk included some professional soldiers amongst their ranks, as well as retired veterans, civilians, and volunteers, while those in Donetsk have been confirmed to include former Berkut special police officers. When asked by The Sunday Telegraph where their weapons had come from, one veteran of the Soviet invasion of Afghanistan nodded at the Russian flag flying from the police station and said: "Look at that flag. You know which country that represents". An insurgent commander in Donetsk, Pavel Paramonov, told journalists he was from Tula Oblast in Russia. In Horlivka, police who defected were commanded by a retired Lieutenant Colonel of the Russian Army, later identified as Igor Bezler. Former Soviet military veteran Vyacheslav Ponomarev, who declared himself mayor of Sloviansk, said that he appealed to old military friends to take part in the militia: "When I called on my friends, practically all of whom are ex military, they came to our rescue, not only from Russia but also from Belarus, Kazakhstan and Moldova."
A former separatist militant corroborated these stories in an interview with Radio Free Europe. He said that fighters, including some Cossack units, arrived from Russia to support the separatists. Another interview with an insurgent from Saint Petersburg was published in Gazeta. He claimed to be fighting voluntarily as part of the Russian Imperial Movement.
In late July 2014, the local support for the militia within the city of Donetsk was estimated to be 70% by a local entrepreneur interviewed by Die Welt. Armed groups affiliated with the Luhansk People's Republic were merged with the Donbas People's Militia on 16 September to form the "United Armed Forces of Novorossiya".
Army of the South-East
The Army of the South-East (, Armiya Yugo-Vostoka) was a pro-Russian militant group that occupied various buildings in Luhansk Oblast. According to The Guardian, their personnel include former members of the disbanded Berkut special police. They were affiliated with the Luhansk People's Republic, but were merged with the Donbas People's Militia on 16 September to form the United Armed Forces of Novorossiya.
Russian Orthodox Army
The Russian Orthodox Army (, Russkaya pravoslavnaya armiya), a pro-Russian insurgent group in Ukraine, originated in May 2014 as part of the insurgency. It reportedly had 100 members at the time of its founding, including locals and Russian volunteers. As fighting between separatists and the Ukrainian government worsened in Donbas, membership rose to 350, and later to 4,000. Notable engagements of the ROA include the June 2014 skirmishes in Mariupol and Amvrosiivka Raion. The headquarters of the ROA is located in an occupied Security Service of Ukraine (SBU) building in Donetsk city. Members swore allegiance to Igor Girkin ("Strelkov"), insurgent and Minister of Defence of the self-declared Donetsk People's Republic. According to the Defence Ministry of Ukraine, the ROA has been in conflict with another pro-Russian militia, the Vostok Battalion, which accused the ROA of looting, and of avoiding combat.