Multiple explosions have been heard in the rebel-held Ukrainian city of Donetsk.
A witness told Reuters they several explosions could be heard in the north of the city on Saturday morning.
It was not clear what caused the explosions. There was no immediate comment from separatists or Ukraine authorities.
The Russia-backed separatist leader of Donetsk, Denis Pushilin, earlier ordered a full mobilisation of rebel troops in the region.
Violence has flared up in recent days in Donetsk and neighbouring Luhansk, the easternmost regions of Ukraine, with several instances of shelling. A bombing struck a car outside an official building in Donetsk on Friday.
Two explosions shook Luhansk early on Saturday. The Luhansk Information Centre said one of the blasts was in a natural gas main and cited witnesses as saying the other was at a vehicle service station.
A mass evacuation of women, children and the elderly from the rebel-held territories to neighbouring Russia has been launched.
Mr Pushilin said they were being moved to safety to avoid shelling by Ukrainian forces. He said, without providing evidence, that Ukraine of preparing to attack soon — an accusation Kiev said was false.
Western leaders have warned that Russia plans to use the conflict in the east as a “false flag” to justify an invasion of Ukraine.
Joe Biden said the separatists were behind the spike in violence as part of a plan to create a “pretext” to bait Ukraine into war.
The president said Russian state media allegations of attacks in eastern Ukraine were “phoney” and “consistent” with Russian schemes to confuse the situation.
Mr Biden said he was “convinced” Vladimir Putin’s forces would launch an invasion “within days” beginning with an attack on the Ukrainian capital of Kiev.
The prospect of a Russian invasion has loomed throughout the year so far, though Moscow denies it has ever been planning to strike.
Ursula von Der Leyen, president of the European Commission, said Moscow's threats towards Ukraine could reshape the entire international system,
She warned Russia that its thinking from “a dark past” could cost the country a prosperous future.
“The world has been watching in disbelief as we face the largest build-up of troops on European soil since the darkest days of the Cold War, because the events of these days could reshape the entire international order,” she told the Munich Security Conference, where leaders have gathered to discuss international security affairs.
