Ukraine's foreign ministry has accused Russia of treating Ukrainian prisoners of war illegally and using them for political purposes, and demanded humane treatment of captured foreigners fighting for Ukraine.
It urged Russia to adhere strictly to the provisions of international humanitarian law, including the 1949 Geneva Conventions which define international legal standards for humanitarian treatment.
"Ukraine condemns the illegal treatment of Ukrainian prisoners of war by the Russian Federation, in particular use of them for its own political purposes," the Ukrainian foreign ministry said on Monday.
"We demand from the Russian side to strictly adhere to the provisions of international humanitarian law, in particular the Geneva Convention relative to the Treatment of Prisoners of War, in relation to servicemen of the Armed Forces of Ukraine who are prisoners of war."
It said "all foreign citizens and stateless persons" fighting on Ukrainian territory for Ukraine's armed forces were voluntarily accepted for military service under Ukrainian law.
"The relevant persons are servicemen of the Armed Forces of Ukraine, to whom, in accordance with the provisions of 1949 Geneva Conventions on the Protection of Victims of War and 1977 Additional Protocol I, the legal status of combatants applies," it said.
"This status, among other things, entitles such persons to be treated as prisoners of war if captured by the enemy power."
In June, a Moroccan and two Britons were sentenced to death by a Russian-backed separatist court in east Ukraine for fighting for Ukraine. They have appealed against the sentences.
Last week, Britain expressed "deep concern" over reports of the death of a British aid worker while in the custody of "a Russian proxy in Ukraine".
Russia did not immediately respond to the foreign ministry statement.
Russia has previously said it is looking into allegations of mistreatment by Russian soldiers.
Ukraine has said it checks all information regarding the treatment of prisoners of war and will investigate any violations and take appropriate legal action.