Ukraine's military has pulled its forces back from defending a vital airport in the east against what is decribed as a column of Russian tanks, as President Petro Poroshenko accused Moscow of "direct and open aggression".
The withdrawal from the civilian airport outside the city of Luhansk was the latest in a string of reverses for Ukrainian forces fighting pro-Russian separatists who Kiev says have the direct support of hundreds of Russian troops and armour.
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An army statement on Monday said Ukrainian paratroopers were engaging a Russian tank battalion near the airport.
Later, Andriy Lysenko, a military spokesman, said: "In the Luhansk direction, Ukrainian forces have received an order and have pulled back from the airport."
Al Jazeera's Paul Brennan, speaking from Mariupol said, "the airport is strategically vital because it was being held by Ukrainian paratroopers and they could easily be supplied by the air."
"It has now fallen to what Ukraine says at least one or maybe four Russian tanks in yet another setback which illustrates changing fortunes in this battle.
"It seems that the inevitable victory now looks impossible for the Ukrainian army, the militia are making big gains which Ukraine says is because of support from Russian regular forces and armoured units."
Poroshenko, speaking at a military academy in Kiev, said Russia's direct involvement in Ukraine's war against the
separatists in eastern Ukraine had tipped the balance on the battlefield and was the main reason for recent setbacks.
"Direct and open 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.
Moscow denies helping the rebels, who have set up "people's republics" in the Russian-speaking east of Ukraine and say they want to be part of the "Russian world" rather than a country which seeks integration into mainstream Europe.
Ukraine's defence minister warned on Monday that a "great war" has broken out with Russia over his country's future that could claim tens of thousands of lives.
"A great war has arrived at our doorstep - the likes of which Europe has not seen since World War II.
"Unfortunately, the losses in such a war will be measured not in the hundreds but thousands and tens of thousands," Valeriy Geletey said.
NATO Secretary-General Anders Fogh Rasmussen said on Monday that Ukraine's political leaders expect a new parliament to abandon the country's non-aligned status after an election next month in a possible prelude to an
application to join the military alliance.
Ukraine's former pro-Russian president Viktor Yanukovich, who was toppled in a revolution this year, had pursued non-aligned status since he took power 2010.
If Ukraine scrapped its non-alliance status after the October 26 vote, NATO would discuss with Kiev "how to move forward," Rasmussen said, but he said it was too early to say how long it might take for Ukraine to join the US-led alliance.
Formal Ukrainian membership in the Western military alliance would come with the full protection of a mutual defence pact with the US superpower, but negotiations to join the alliance can take years as potential members have to meet a series of political and military criteria.
"I understand that the current political leadership in Kiev envisages that the new parliament after the parliamentary election in Ukraine will change the legislation on non-alliance status," Rasmussen told a news conference in Brussels.
"We have to wait for elections, we have to wait for the legislative procedure in the new Ukrainian parliament," he said.
If Ukraine's parliament passed legislation removing Ukraine's non-aligned status, Rasmussen said he said he would then expect Ukraine to raise the issue with NATO.
NATO denied Ukraine a fast track towards membership in 2008 when a previous pro-European government in Kiev tried to pursue closer ties with the alliance.
Until this government's apparent switch last week, Poroshenko, who was elected to succeed Yanokovich in June, had also said he did not back joining NATO because there was no popular support for it.
Poroshenko hopes the election will clear out many of the "old guard" who supported Yanukovich and produce a coalition able to push through vital economic and political reform after years of corrupt misrule and malpractice.