Ukraine's military says six of its soldiers have been killed and another 14 wounded in fighting over the past day.
If confirmed, it would be one of the highest casualty tolls reported in a single day since an often-violated ceasefire was called more than three months ago.
Colonel Andriy Lysenko, a military spokesperson, did not give specifics on Saturday as to where the deaths occurred but he said fighting was especially heavy in the vicinity of the destroyed airport in Donetsk.
Donetsk is the largest city under the control of pro-Russian separatists in eastern Ukraine.
Eastern Ukraine's busiest air hub turned into a symbolic war prize that Ukraine lost hundreds of soldiers defending until January.
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Neighbourhoods around the Donetsk airport suffer continual shelling to this day.
Lysenko said separatist fighters were using tanks and heavy artillery and that they fired on residential buildings in Vodyane and Avdiivka, both of which are about 5km from the airport.
"The situation near the airport is particularly restive," he said. "Our enemies have thrown tanks, infantry combat vehicles and artillery into battle."
Separatists in Donetsk accuse the Ukraine military of shelling the very regions mentioned by Lysenko.
They also charge government forces with committing "99 ceasefire violations" since Friday.
In Saturday's other developments, Ukraine protested to Russia after mobs vandalised its embassy in Moscow and its consulate in Rostov-on-Don overnight.
The move came a day after Russia made a similar protest to Ukraine over damage to the Russian consulate in Kharkiv.
Russian media reported earlier on Saturday that about 100 people had thrown eggs at Ukraine's Moscow embassy and that attackers had broken windows with bricks and baseball bats at Ukraine's consulate in Rostov-on-Don in southern Russia.
RIA news agency quoted the Ukrainian consul in Rostov as saying that the consulate may need to be closed for several days as many windows had been broken and office equipment damaged.
"We demand from the Russian authorities an immediate comprehensive investigation, strict punishment of those responsible and compensation for material damage," the Ukrainian statement said.
On Friday Russia's foreign ministry published a note of protest demanding an investigation into damage to its consulate in Kharkiv, which was the target of eggs and paint thrown by vandals on Thursday.