Ukrainian forces have withdrawn from the airport of the eastern city of Luhansk following clashes with pro-Russian rebels, military officials say.
The announcement came after the two sides were engaged in all-night fighting.
Russian Foreign Minister Sergei Lavrov has urged negotiators meeting in the Belarus capital, Minsk, to make an “immediate ceasefire” their priority.
Some 2,600 people have died in eastern Ukraine since fighting began in April.
The conflict broke out after Russia’s annexation of Ukraine’s southern Crimea peninsula in March.
The rebels have been gaining ground on Ukrainian forces in recent days, in both the Luhansk and Donetsk regions, and further south around the port of Mariupol.
Ukraine and the West blame Russian military support for the rebel advances, saying armoured columns have crossed the border – allegations rejected by Russia’s President Vladimir Putin.
He has accused European leaders of ignoring the Ukrainian army “directly targeting civilians”.
On Sunday, he said the issue of “statehood” for eastern Ukraine needed to be discussed to ensure the interests of local people were “definitely upheld”.
In other developments:
- President Putin urged the European Union to show “common sense” and not to engage in mutually destructive sanctions, in his first reaction to the threat of new sanctions over Ukraine
- German Chancellor Angela Merkel acknowledged that further sanctions against Russia could aversely affect her country’s economy, but said “accepting Russia’s behaviour [was] not an option”
- Russia’s energy minister said Russia and Ukraine had agreed to hold talks on 6 September to resolve the gas crisis, adding that the EU had yet to confirm whether it would send a representative.
‘No military intervention’
Ukraine’s security council confirmed on Monday that its troops had withdrawn from Luhansk airport “in an organised manner”.
Clashes are said to still be taking place near the airport of the city of Donetsk, with separatists claiming that two Ukrainian platoons have surrendered.
There were also reports of an attack on a Ukrainian patrol vessel in the Azov Sea on Sunday night.
Meanwhile, Ukrainian and Russian officials are due to hold talks with separatist rebels and international monitors in Minsk.
The meeting of the so-called Contact Group will include representatives from the Organization for Security and Co-operation in Europe.
Mr Lavrov said he was counting on the talks to focus on “agreeing an immediate and unconditional ceasefire”
He also insisted there would be “no military intervention” from Russia in Ukraine.
“We are for an exclusively peaceful resolution of that most serious crisis, that tragedy,” Mr Lavrov told students in Moscow.
Last week’s first direct talks between President Putin and his Ukrainian counterpart, Petro Poroshenko, did not lead to any major breakthrough.
Mr Poroshenko on Monday again accused Russia of “direct and open aggression” in eastern Ukraine.
But Mr Putin earlier said Russia could not “stand aside when people are being shot at almost at point blank”.
He described the rebels’ actions as “the natural reaction of people who are defending their rights”.
Mr Putin’s comments came after the EU gave Russia a one-week ultimatum to reverse course in Ukraine or face more sanctions.
The EU and US have already imposed asset freezes and travel bans on many senior Russian officials and separatist leaders in eastern Ukraine.