Russian forces have accelerated attacks on Ukraine in recent weeks, with the Kremlin vowing to retaliate over a brazen attack on its air bases last weekend.

The second round of negotiations between the Russians and Ukrainians on a possible ceasefire has ended after about an hour, according to Russian news agencies. State news agency TASS and the Interfax agency cited their own sources as saying the talks were over and that there were no plans to continue the talks later in the day.

Ukrainian and Russian delegations met again in Istanbul on Monday to take part in the next round of U.S.-brokered peace talks.