Ukrainian President Petro Poroshenko has cancelled a visit to Turkey, citing "Russian troop deployments" in the east of the country, reports GHN based on BBC.
His announcement came as pro-Russia rebels took the coast town of Novoazovsk and threatened the strategic port city of Mariupol.
Russia said no forces were crossing Ukraine's border "at any point".
At least 2,119 people have been killed since fighting erupted in April between Ukrainian forces and separatists.
The latest rebel successes constitute the opening of a new front in the conflict.
'Sharp aggravation'
On Thursday, Mr Poroshenko said his place was in Kiev in view of the deterioration in the situation in Donetsk region.
"I have made a decision to cancel my working visit to the Republic of Turkey due to the sharp aggravation of the situation in Donetsk region... as Russian troops were actually brought into Ukraine," Mr Poroshenko said in a statement.
He added that he was calling a meeting of the Ukrainian security council, and that Ukraine would initiate an emergency session of the United Nations Security Council to discuss the crisis.
Meanwhile Prime Minister Arseny Yatsenyuk said Russia had "unleashed a war in Europe" and said the world should take "effective steps".