Russia and Ukraine held a second round of peace talks where they agreed to exchange more prisoners and to return the bodies of 12,000 dead soldiers.

Russia and Ukraine wrangled over details of a large prisoner swap, with officials in Kyiv rejecting claims by a Russian negotiator that they were dragging their feet on the planned exchange.

A large-scale prisoner exchange between Russia and Ukraine -- the only concrete outcome of peace talks -- was in doubt Saturday after Moscow and Kyiv accused each other of delaying

The attack on Friday, which killed at least four people nationwide, came two days after Russia vowed retaliation for an audacious drone assault on its strategic bomber bases.

Russia's Defence Ministry said on Friday that its forces had carried out a massive and successful strike on military and military-related targets in Ukraine overnight in response to what it called Ukrainian "terrorist acts" against Russia.