Ukrainian president Petro Poroshenko has condemned Russia for what he says are Russian troop movements into the south-east of Ukraine.
Mr Poroshenko cancelled an official trip to Turkey and convened an urgent meeting of his security advisors to deal with what he called the rapidly deteriorating situation in the embattled Donetsk region.
“I have made a decision to cancel my working visit to the Republic of Turkey due to sharp aggravation of the situation in Donetsk region, particularly in Amvrosiivka and Starobeshevo, as Russian troops were actually brought into Ukraine,” Mr Poroshenko said in a statement.
A human rights advisor to Russian president Vladimir Putin has also said an invasion is taking place, according to reports.
The international community earlier issued a warning to Russia, who the US ambassador said was now “directly involved” in fighting in the region.
“An increasing number of Russian troops are intervening directly in fighting in Ukrainian territory,” Geoffrey Pyatt wrote on Twitter, adding that Moscow had sent its “newest air defence systems”.
Separatists backed by Russian soldiers have entered the town of Novoazovsk in south-eastern Ukraine, a fighter from the so-called Azov battalion, which supports the Ukrainian army, said.
“There is military equipment in Novoazovsk which came across the border two days ago from Russia,” the fighter said.
“The equipment is carrying the flags of the DNR [Donetsk People’s Republic] rebels but they are regular Russian forces.”
Joining the US ambassador in condemning the move was French president Francois Hollande, who warned it would be “unacceptable” if Russian troops were found to be operating on Ukrainian territory, as Western intelligence also suggests.
“If it turns out that there are Russian soldiers present on Ukrainian soil, it would be intolerable and unacceptable,” he said.
Ukraine’s prime minister Arseny Yatseniuk has appealed to the United States as well as European Union and G7 countries to freeze Russian assets until its forces withdraw from Ukrainian territory.
The latest developments follow the deaths of 11 civilians and the wounding of 22 others in mortar bombardments in east Ukraine’s main rebel bastion of Donetsk in the past 24 hours.