DONETSK, Ukraine - Washington and Kiev said Thursday that Russian troops were actively involved in the fighting tearing apart eastern Ukraine, raising fears of a direct military confrontation on Europe's eastern flank.
Ukrainian President Petro Poroshenko scrapped a trip to Turkey and called an urgent meeting of his security chiefs over what a top official described as a "direct invasion" by Russia.
The latest rapid-fire developments have sent alarm bells ringing in the United States and in Europe, where EU leaders are set to discuss the crisis on their doorstep at a weekend summit. Kiev said Russian soldiers had seized control of a key southeastern border town and a string of towns in an area where fighting had been raging.
US officials accused Russian troops of being behind a lightning counter-offensive that has seen pro-Moscow rebels seize swathes of territory from government forces, dramatically turning the tide in the four-month conflict. "An increasing number of Russian troops are intervening directly in fighting on Ukrainian territory," the US ambassador to Kiev Geoffrey Pyatt wrote on Twitter.
"Russia has also sent its newest air defence systems including the SA-22 into eastern Ukraine and is now directly involved in the fighting," he said. NATO said at least 1,000 Russian troops were on the ground. But Russia swiftly denied the allegations, saying none of its soldiers were in Ukraine.
Kiev called on the West for help after a counter-offensive from the southeast border with Russia appeared to smash through an army blockade around the separatist stronghold of Donetsk and threaten the government-held port city of Mariupol.
The gains by the separatist fighters come after weeks of government offensives that have seen troops push deep into the last rebel bastions in Ukraine's industrial heartland.
Ukraine's Prime Minister Arseniy Yatsenyuk blasted Russian President Vladimir Putin for having "deliberately unleashed a war in Europe" and called for an emergency UN Security Council meeting, a called echoed by Lithuania.
- Russian soldiers 'on holiday' -
A top rebel leader, Alexander Zakharchenko, admitted Wednesday that Russian troops were fighting alongside his insurgents, but said they were on "holiday" after volunteering to join the battle.
The spiralling tensions come after Poroshenko and Putin on Tuesday held their first meeting in three months but failed to achieve any concrete breakthrough despite talk of a peace roadmap.
French President Francois Hollande warned it would be "intolerable and unacceptable" if Russian troops were on the ground after German Chancellor Angela Merkel demanded an explanation from Putin over the reports.
The EU said it was "extremely concerned" about the escalating developments on its eastern flank and called on Russia to end its "border hostility".
The United States and the EU have already imposed a series of punishing sanctions on Russia over the crisis, the worst standoff between Moscow and the West since the Cold War.
Kiev said Russian troops on Wednesday seized control of the key border town of Novoazovsk and a string of surrounding villages along the southeastern strip of the frontier, where fighting had raged for days.
A volunteer pro-Kiev commander said government troops were surrounded in the key transport hub of Ilovaysk some 50 kilometres southeast of Donetsk and were running out of ammunition.
Ukraine's military also claimed a Russian battalion had set up its headquarters near a village in the same area.
AFP journalists on Wednesday found signs of a hurried retreat by Ukrainian forces after they appeared to have abandoned a key road leading southeast from Donetsk to the Russian border.
Russian has repeatedly denied it is involved in the insurrection in the former Soviet state, and Foreign Minister Sergei Lavrov said Wednesday the Kremlin was "not interested in breaking up" Ukraine.
AFP journalists reported heavy shelling in Donetsk on Thursday, with local authorities saying 11 civilians had lost their lives in 24 hours.
The United Nations estimates the conflict has killed over 2,200 people and forced more than 400,000 to flee since April.
On Wednesday, Yatsenyuk said it was time for NATO to act, calling for "practical help and... crucial decisions" when it holds a summit in Wales next week.
Russia vehemently opposes closer ties between Ukraine and NATO. Concerns that Kiev could be drawn closer into the Western security alliance are seen as a key motivation behind Russia's actions in recent months.
Ukraine's ambassador to the EU also appealed for "large-scale" military assistance from Brussels and called for the EU Council to meet Saturday on the crisis.
Following his crunch meeting with Putin on Tuesday Poroshenko had claimed that all sides "without exception" had backed a Kiev peace plan for war-torn east Ukraine.
But Putin had insisisted that discussing any ceasefire was not Moscow's "business" but an internal Ukrainian affair.
But opposition media in Russia reported on hushed-up funerals for two elite paratroopers, suggesting they had been killed in action in Ukraine.
A group of wives and mothers of Russian paratroopers plan to hold a rally Thursday.