Pro-Russian separatist rebels have control over approximately 300 Ukrainian prisoners of war in war-torn eastern Ukraine, some of whom were purportedly taken across the border into Russia, a Ukrainian official said. Separatist forces in the region have been repeatedly accused of mistreating captured soldiers during the conflict, which began in February 2014.

Separatist forces have transported Ukrainian POWs to several major Russian cities, including Moscow and St. Petersburg, Ukrainian Defense Ministry spokesman Yuri Tandyt said, as English-language Ukrainian television station Ukraine Today reported. However, Ukrainian officials negotiated the return of hundreds of other captured soldiers, he said.

The Ukrainian government and members of the NATO military alliance have accused Russia of providing support to separatist rebels during the conflict. Moscow has denied the allegations.

Human rights group Amnesty International accused pro-Russian rebels earlier this month of carrying out summary executions of captured Ukrainian soldiers in eastern Ukraine. The organization said it obtained footage that showed dead Ukrainian soldiers with “bullet wounds to their heads and upper bodies,” as well as eyewitness accounts from individuals who purportedly saw a rebel leader execute a Ukrainian POW.

“The torture, ill treatment and killing of captured, surrendered or wounded soldiers are war crimes,” Denis Krivosheev, deputy director of Europe and Central Asia for Amnesty International, said in an April 9 statement. “These claims must be promptly, thoroughly and impartially investigated, and the perpetrators prosecuted in fair trials by recognized authorities.”

The Kyiv Post obtained audio earlier this month in which Arseniy Pavlov, leader of separatist military organization “Sparta Battalion,” purportedly confessed he executed 15 Ukrainian soldiers. If authenticated, the confession could constitute evidence of a war crime.

More than 6,000 people have died as a result of the fighting in Ukraine, the United Nations estimated in March. A cease-fire between Ukrainian government forces and pro-Russian rebels has mostly stemmed fighting since February, thought sporadic clashes have occurred.