Russian-backed rebels in eastern Ukraine said on Monday two civilians were killed in shelling by the Kyiv government forces, Russia's RIA news agency reported.
RIA cited representatives of the self-proclaimed Luhansk People's Republic as saying that the shelling occurred late on Sunday.
A blast was also heard earlier on Monday in the center of the city of Donetsk, which is held by Russian-backed separatists in eastern Ukraine, a Reuters witness said. The origin of the blast was unclear.
Kyiv and the rebels, at war for the last eight years, have accused each other of increased ceasefire violations over the last few days and the use of heavy weaponry such as mortars and artillery. Russia, which has massed over 100,000 troops near Ukraine's borders says it is concerned about the increase in hostilities.
US President Joe Biden has accepted in principle a summit with Russia's Vladimir Putin over the Ukraine crisis after the two countries' foreign ministers meet next week and if an invasion has not occurred, the White House said on Sunday.
Dozens of demonstrators calling for peace in Ukraine marched through parts of downtown Washington, DC, on Sunday. The march ended at the White House, where US President Joe Biden had convened the National Security Council earlier in the day and spoken to French President Emmanuel Macron about the situation in Ukraine.
Satellite images show multiple new field deployments of armored equipment and troops from Russian garrisons near the border with Ukraine, a private US company said on Sunday, in what its director said indicated increased military readiness.
The new activity comes as Russia extended military drills in Belarus that were due to end on Sunday. US Secretary of State Antony Blinken said the move has made him more worried about an imminent Russian invasion of Ukraine.
The images released by US-based Maxar Technologies, which has been tracking the buildup of Russian forces for weeks, could not be independently verified by Reuters.
"This new activity represents a change in the pattern of the previously observed deployments of battle groups (tanks, armored personnel carriers, artillery and support equipment)," Maxar said in a release on Sunday.
Several large deployments of battle groups had been observed around the military garrison at Soloti, Russia, as of February 13 according to satellite images, Maxar said.
Images on Sunday showed most of the combat units and support equipment at Soloti had departed. Extensive vehicle tracks and some convoys of armored equipment were seen throughout the area, the company said.
Some equipment has also been deployed east of nearby Valuyki, Russia, in a field approximately 15 km (9 miles) north of the Ukrainian border. A number of new field deployments are also seen northwest of the Russian city of Belgorod, Maxar said.
Based on the tracks and the snow, the activity appeared to be recent, Stephen Wood, senior director at Maxar News Bureau, told Reuters.