Evidence that Russia-backed separatists in eastern Ukraine killed soldiers in their captivity execution-style has been forwarded to the International Criminal Court (ICC) in The Hague.

The Ukrainian Prosecutor-General's office said it had photo and video evidence relating to the killings of nine servicemen by the separatists during fighting at Ilovaisk in 2014 and Debaltseve in 2015.

In a press release, the Ukrainian Prosecutor-General said its evidence suggests the killings were carried out by fighters of the so-called Cossack Union Don Army Oblast.

The material was prepared with assistance from the Ukrainian Helsinki Human Rights Union. It includes photographic and video evidence showing that the Ukrainian soldiers had been captured, as well as witness testimony and documents confirming they were killed while prisoners.

Amnesty International in 2015 reported it had proof of "execution-style killings" by Russia-backed armed groups in eastern Ukraine, where more than 13,000 have died in fighting since April 2014, a month after Russia illegally annexed Ukraine’s Crimean Peninsula.

Fighting in Ilovaisk in August-September 2014 and Debaltseve in February 2015 was especially fierce with Western intelligence and reporters on the ground saying the separatists were backed by regular Russian armed forces.

The open-source research group Forensic Architecture reported in August that sophisticated Russian military hardware had been employed during the fighting in Ilovaisk, including Russian T-72B3 tanks, depicted in satellite imagery in and around Ilovaisk at that time.

Russia has repeatedly denied providing arms, financing, or any other support to the separatists in eastern Ukraine, despite much evidence to the contrary.

In its 2015 report, Amnesty International said: "The torture, ill-treatment and killing of captured, surrendered or wounded soldiers are war crimes."

In its December 5 statement, the Ukrainian Prosecutor-General’s office accused Moscow of employing illegal armed groups not only to gain military advantage, but to "carry out serious violations of international humanitarian law."

The ICC ruled in November 2016 that the war in eastern Ukraine is “an international armed conflict between Ukraine and the Russian Federation.”

Critics say Russia encouraged separatism and fomented unrest across much of Ukraine after Moscow-friendly President Viktor Yanukovych fled the country in February 2014 in the face of mass protests known as the Maidan.

The protests erupted in the fall of 2013, when Yanukovych abruptly abandoned plans to sign a pact tightening Ukraine's ties with the European Union and called for closer economic interaction with Russia instead.

Ukraine has approached the ICC to examine the worst crimes perpetrated during and after the Maidan as well as during the fighting in eastern Ukraine.