Pro-Russian separatists and government forces clashed near the main airport in Donetsk on Monday after a counter-offensive on Sunday by Ukrainian troops to reclaim lost ground and the Kiev military said three Ukrainian soldiers had been killed.
Military spokesman Andriy Lysenko said: "The (separatist) fighters of the Donetsk and Luhansk People's republics are continuing to storm Donetsk airport and fire on our positions.
"This is not simply a strategic objective. It is a launching pad from which the terrorists can begin a new offensive. We will not surrender the airport," he said, adding that three soldiers had been killed and 66 others wounded in the past 24 hours.
Ukrainian troops have recaptured almost all the territory of Donetsk airport in eastern Ukraine they had lost to separatists in recent weeks and thousands gathered in Kiev for a state-sponsored peace march.
Sunday's offensive brought fighting close to the industrial city of Donetsk, center of a pro-Russian rebellion, while shelling intensified in other parts of the region known as "Donbass".
With attempts to restart peace talks stalled, pro-Russian rebels have stepped up attacks in the past week and casualties have mounted, including 13 civilians killed in an attack on a passenger bus, which Kiev blamed on the separatists.
Military spokesman Andriy Lysenko said the army's operation had returned battle lines near the airport to the previous status quo and thus not violated the 12-point peace plan agreed with Russia and separatist leaders last September in Minsk.
"We succeeded in almost completely cleansing the territory of the airport, which belongs to the territory of Ukrainian forces as marked by military separation lines," he said.
Another military spokesman said the firing was continuing early on Monday: "There is firing going on in the airport area and also at various parts of the line of separation. The situation has not particularly improved."
Kremlin spokesman Dmitry Peskov said Moscow was concerned by what he called an escalation by Ukrainian forces that did not contribute to peace efforts.
He later said Ukrainian President Petro Poroshenko had rejected a peace plan contained in a letter Russian President Vladimir Putin sent him on Thursday.
"In recent days, Russia has consistently undertaken efforts as an intermediary in regulating the conflict," Peskov said, according to TASS news agency.
It said Putin's letter included a concrete plan for both sides to withdraw heavy artillery.
Russian television channel NTV published the letter on Sunday evening. In it, Putin proposed "urgent measures for the cessation of mutual shelling, and also the rapid withdrawal by the sides in the conflict of means of destruction with a calibre higher than 100 mm".
In an overnight statement, the Ukrainian Foreign Ministry called on Russia to sign with Ukraine a timetable for implementing the Minsk agreement which included establishing Ukrainian control over the joint border, withdrawal of foreign forces and heavy military equipment from the region.
"We are convinced that this will allow us quickly to cease fire and protect the civilian population from the firing from (separatist) fighters for the past four months," it said.
"Ukraine steadfastly supports all the points of the Minsk agreements and demands that Russia does also," it said.
REUTERS