Russia’s Defense Ministry said that Russia and Ukraine exchanged 185 prisoners of war each in line with agreements reached in Istanbul.

It added that 20 civilians were also returned to Russia as part of the swap.

The released Russian servicemen are currently in Belarus, where they are receiving psychological and medical assistance, according to the ministry. Before boarding the bus at an undisclosed location they held Russian flags and chanted.

Meanwhile, on the battlefield, Russia’s sustained bombardment of Ukraine's power grid is deepening concerns about the safety of the country's nuclear facilities after a drone knocked out power for more than three hours to the site of the 1986 Chernobyl nuclear disaster and as Europe’s biggest atomic power plant remains disconnected from the grid, officials said.

Both Chernobyl and the Russian-occupied Zaporizhzhia Nuclear Power Plant are not working but they require a constant power supply to run crucial cooling systems for spent fuel rods in order to avoid a potential nuclear incident.

A blackout could also blind the radiation monitoring systems, installed to boost security at Chernobyl and operated by the U.N. nuclear watchdog, the International Atomic Energy Agency.

Denmark warned on Wednesday at a European Union leaders' summit in Copenhagen that Europe is in the middle of a hybrid war waged by Russia and that the continent must arm itself.

The summit came after a spate of troubling drone incidents at Danish airports and military bases over the last week. Ahead of the meeting, a special radar system was set up at Copenhagen airport to help keep watch. Unidentified drones forced the closure of the airfield a week ago, causing major disruptions.

France, Germany, the Netherlands, Sweden and the U.K. also sent aircraft, ships and air defense systems to Denmark ahead of the talks. Ukraine’s armed forces have dispatched a mission to the Nordic country for joint exercises, sharing its expertise on combating Russian drones.