Ukraine's president says a new prisoner exchange with Russia is ongoing and will take days, as both sides continue launching hundreds of attack drones.

Russia and Ukraine exchanged prisoners of war under the age of 25 on Monday in emotional homecoming scenes, the first step in a series of planned prisoner swaps that could become the biggest of the war so far.

The warring sides also exchanged prisoners in one of the few results of talks last week, and Russia reported new attacks on the front line.