The United States on Thursday accused Russia of being "directly involved" in fighting in war-torn east Ukraine, after rebels appeared to seize swathes of territory from retreating government forces.
"An increasing number of Russian troops are intervening directly in fighting on Ukrainian territory," US ambassador to Kiev Geoffrey Pyatt wrote on Twitter.
"Russia has also sent its newest air defense systems including the SA-22 into eastern Ukraine and is now directly involved in the fighting," he said.
After weeks of government offensives that have seen troops push deep into the last rebel bastions, the tide appeared to be turning once again in the four-month conflict, prompting a nervous government in Kiev to call on NATO for help.
AFP journalists on Wednesday saw no signs of government troops on a road southeast of the rebel-held city of Donetsk, while a pro-Kiev volunteer commander wrote that government forces were surrounded in the key transport hub of Ilovaysk.
There has been increasing concern in Kiev and the West of Russia's direct involvement in the conflict -- a charge Moscow has repeatedly denied.
Poland's Prime Minister Donald Tusk said NATO and Polish intelligence have evidence of regular Russian army units operating in Ukraine.
French President Francois Hollande warned on Thursday it would be "intolerable and unacceptable" if Russian troops were on the ground in Ukraine.
Ukraine's government claimed on Wednesday that a battalion of Russian soldiers had set up a military headquarters near the village of Pobeda, around 50 kilometres (30 miles) southeast of Donetsk.
A top rebel leader on Wednesday admitted Russian troops were fighting alongside his insurgents, but said they all volunteered to spend their "holidays" battling for the rebels.
"Many Russian soldiers are joining us who would prefer to spend their holidays not at the beach but in the ranks of their brothers fighting for the freedom of Donbass," Alexander Zakharchenko, the prime minister of the self-proclaimed Donetsk People's Republic, said in an online statement.
Russian Foreign Minister Sergei Lavrov said the Kremlin was "not interested in breaking up" Ukraine, but said Russia will send more aid convoys to Ukraine "in the nearest future", despite complaints from Kiev they breach its sovereignty.
Local authorities in the rebel bastion of Donetsk said shelling killed 11 civilians in 24 hours, the latest casualties in over four months of fighting that cost the lives of over 2,200.
Government troops surrounded
In the town of Starobesheve, about 30 kilometres southeast of Donetsk, there were signs of a hasty retreat by Ukraine's army on Wednesday including an abandoned tank.
Locals told AFP the troops left on Monday after shelling from the direction of the Russian border about 30 kilometres away.
Ukraine's military conceded that "militants together with Russian occupants" had taken control of Starobesheve, as well as a string of villages near Novoazovsk, a town on the Azov Sea where clashes had been raging for days.
Meanwhile, Commander Semen Semenchenko, head of the pro-Kiev volunteer "Donbass battalion," posted on Facebook that troops were surrounded by rebels, running out of ammunition and unable to evacuate the injured.
Call for NATO aid
Ukraine's Prime Minister Arseniy Yatsenyuk said it was time for NATO to act when the alliance holds a summit in Wales next week.
"We expect our Western partners and the alliance to provide practical help and take crucial decisions at the summit in September," he said.
Russia vehemently opposes closer ties between Ukraine and NATO. Concerns that Kiev could be drawn closer into the Western security alliance are seen as the main motivation behind Russia's actions in recent months.
NATO chief Anders Fogh Rasmussen said in an interview published on Wednesday that the alliance was preparing a rapid response unit to deploy troops swiftly in eastern Europe.
The spiralling tensions come after Ukrainian president Petro Poroshenko and Russian leader Vladimir Putin on Tuesday held their first meeting in three months but failed to achieve any concrete breakthrough.
The Ukrainian president said all sides had "without exception" agreed to his peace plan, and that he and Putin had discussed the "necessity of closing Ukraine's borders" to prevent the movement of "equipment, mercenaries, and ammunition".
But Putin again insisted Russia had no role in the fighting, insisting discussing any ceasefire was not Moscow's "business".
He played down reports that 10 Russian paratroopers had been captured inside Ukrainian territory, backing his military's claims that they had strayed across the border by accident.
But opposition media in Russia reported on hushed-up funerals for two elite unit paratroopers, suggesting they had been killed in action in Ukraine.
A group of wives and mothers of Russian paratroopers announced they would hold a rally Thursday in the central Russian town of Kostroma demanding authorities pull their loved ones from Ukraine.