Kiev/Moscow: Ukrainian President Petro Poroshenko accused Russia on Monday of "direct and undisguised aggression" which he said had radically changed the battlefield balance as Kiev's forces suffered a further reverse in their war with pro-Moscow separatists.
In the latest in a string of setbacks in the past week, Ukraine's military said it had pulled back from defending a vital airport in the east of the country, near the city of Luhansk, where troops had been battling a Russian tank battalion.
Poroshenko said in a speech there would be high-level personnel changes in the Ukrainian armed forces, whose troops fled a new rebel advance in the south which Kiev and its Western allies say has been backed up by Russian armoured columns.
Russian President Vladimir Putin, who called on Sunday for immediate negotiations on the "statehood" of southern and eastern Ukraine, blamed Kiev's leadership for refusing to enter into direct political talks with the separatists.
Poroshenko repeated Kiev's belief that Russian forces are helping the rebels to turn the tide of the war.
"Direct and undisguised aggression has been launched against Ukraine from a neighbouring state.
"This has changed the situation in the zone of conflict in a radical way," he said in his speech at a military academy in Kiev.
Defence Minister Valery Heletey added on his Facebook page that Ukraine no longer faced a threat from separatists but outright war with Russian troops.
"Unfortunately, in such a war, the losses will be numbered not in their hundreds, but in thousands, even tens of thousands," he said.
"We must refrain from panic and show that Ukrainians are not about to surrender."
In the Belarussian capital, Minsk, separatists sat down for preliminary peace talks with Ukraine, saying they would be prepared to stay part of Ukraine if they were granted "special status", according to Russian news agencies.
But they said one of their key conditions would be for Kiev to immediately end its military offensive.
The separatists' demands did not appear, at first sight, to be acceptable to Kiev since they would leave the rebels in control of the territories of Ukraine's industrialised east and exercising a trade policy tilted towards Russia and away from integration with the European Union, which is Kiev's key aim.
Military spokesman Andriy Lysenko said Ukrainian forces had pulled back from the airport near Luhansk.
However, they had destroyed seven Russian tanks and identified a major build-up of Russian forces to the north and south of the city.
"According to our operational data, there are no fewer than four (Russian) battalion-tactical groups in Ukraine," he told reporters, adding that each one comprised 400 men.
Speaking during a visit to Siberia, Putin repeated his call for talks.
"The current Kiev leadership does not want to carry out a substantive political dialogue with the east of its country," state news agency Itar-Tass cited him as telling journalists.
Putin also said the separatists were trying to force Ukrainian troops from their current positions where they were firing on civilian targets.
"The aim of the militia fighters is to push away these armed forces and their artillery to not give them the possibility to shoot on residential areas," he said.
Kiev has clung to a non-aligned status as it tried to steer between two dominant powers — Russia to the east and Europe to the west.
However, Nato Secretary-General Anders Fogh Rasmussen said Ukraine's political leaders expect a new parliament to abandon this status after an election next month in a possible prelude to an application to join the Western alliance.
Putin made his statehood remarks two days after comparing the Kiev government with Nazis and warning the West not to "mess with us".
On Sunday, Putin's spokesman said his call for talks on the statehood of southern and eastern Ukraine did not mean Moscow now endorsed rebel calls for independence for territory they have seized.