Ukrainian President Petro Poroshenko has cancelled a visit to Turkey, citing “Russian troop deployments” in the east of the country.
Mr Poroshenko said his place was in Kiev in view of a sharp deterioration in the situation in Donetsk region.
His announcement came as pro-Russian rebels captured the seaside town of Novoazovsk and threatened to take the strategic port city of Mariupol.
The rebel successes constitute the opening of a new front in the conflict.
Mr Poroshenko said he was calling a meeting of the Ukrainian security council.
He added that Ukraine would initiate an emergency session of the United Nations Security Council to discuss the crisis.
“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… as Russian troops were actually brought into Ukraine,” Mr Poroshenko’s statement said.
Ukrainian officials have repeatedly said that Russia has been sending troops and equipment to the rebels, but Russia has denied arming or covertly supporting them.