Pro-Russian separatists renewed attacks on Ukrainian forces at an airport complex in the east on Monday after Kiev launched a mass operation to reclaim lost ground there that Russia called a "strategic mistake."
Ukrainian officials said three soldiers had been killed and 66 wounded over the past 24 hours, during which they said they had returned battle lines at the airport to the status quo under a much-violated international peace plan.
Russia expressed concern at what it called escalation by Kiev and published its own peace plan on Monday in the form of a letter from President Vladimir Putin to Ukrainian President Petro Poroshenko, which Russia said Poroshenko had rejected.
"It's the biggest, even strategic mistake of the Ukrainian authorities to bank on a military solution to the crisis," the Interfax news agency quoted Deputy Foreign Minister Grigory Karasin as saying. "This may lead to irreversible consequences for Ukrainian statehood."
Ukrainian officials have insisted Moscow stick to the 12-point peace plan agreed in Minsk in September, which they say was not violated by its airport counter-offensive, launched after troops had appeared to be pinned down inside the complex.
Ukrainian military spokesman Andriy Lysenko said the situation was still very tense around the airport, which has symbolic value for both sides, and separatists continued attacks on government forces there and elsewhere in the east.
A military source said that up to 25 Russian tanks had crossed the border near Luhansk.
Since plans for another round of peace talks last week were abandoned, fighting has flared up again in Ukraine, whose Crimean peninsula was annexed by Russia in March last year, prompting a crisis with the West, which has imposed sanctions.
In Brussels, European Union foreign ministers said now was not the time to ease the economic sanctions against Russia despite conciliatory proposals from the EU's foreign policy chief Federica Mogherini.
Mogherini had suggested that member states could start talking to Russia again on a range of issues if Moscow implemented peace agreements, but hawkish states such as Lithuania said this would send the wrong message to Russia's Putin that the EU's resolve was cracking.
"I don't think that we now should think how to re-engage. Russia should think how to re-engage," Lithuanian Foreign Minister Linas Linkevicius told reporters.
Apart from calling for a ceasefire, the Minsk agreement called for the withdrawal of armed groups and foreign fighters as well as military equipment — meaning, for Kiev, weapons and rocket systems which it says Moscow is supplying to the rebels.
Despite what Kiev and the West say is incontrovertible proof, Russia denies its troops are involved or that it is funneling military equipment to the separatists.
Putin's letter called for urgent moves to withdraw large-caliber weapons from the conflict zone. "This is now an absolute priority," said a Russian Foreign Ministry statement.
Ukraine's Foreign Ministry hit back, calling for Moscow to show its readiness to comply with the Minsk agreement by signing a timetable for implementing its main points.
"It is very important that a concrete plan is signed for fulfilling all, without exception, the points of the Minsk agreements, and not just those that Russia or the terrorists like," Foreign Ministry spokesman Yevhen Perebyinis said on TV channel 112.
Military spokesman Lysenko said Russia was continuing to send significant quantities of military equipment into Ukraine.
The World Health Organization says more than 4,800 people have been killed.