Kiev accuses Russia of deploying troops in Ukraine

Ukrainian President Petro Poroshenko  cancelled a visit to Turkey, citing "Russian troop deployments" in the east of the country.

Poroshenko said his place was in Kiev in view of a sharp deterioration in the situation in Donetsk region. His statement came as pro-Russia rebels took the coast town of Novoazovsk and threatened the strategic port city of Mariupol, in the region's south.

Meanwhile, a pro-Russia rebel leader has said that Russian soldiers, on leave from their posts, are fighting Ukrainian troops alongside the country's separatists in eastern Ukraine, the website of a Russian state television channel has reported.

"Among us are fighting serving soldiers, who would rather take their vacation not on a beach but with us, among brothers, who are fighting for their freedom," said Zakharchenko in an interview posted on Thursday on Vesti.ru.

The rebel successes constitute the opening of a new front in the conflict. 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," Poroshenko's statement said.

Government forces had made significant advances against the separatists in recent weeks, but these gains seem in doubt with rebels now operating in two distinct areas of Donetsk region.

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.

Arseniy Yatseniuk, Ukraine's prime minister, appealed to the United States, European Union and G7 countries on Thursday to freeze Russian assets until Russian forces withdraw from Ukrainian territory.

Meanwhile, Washington on Thursday accused Moscow of being "directly involved" in fighting in Ukraine.

Geoffrey Pyatt, the US ambassador to Kiev, wrote on Twitter that "Russia has also sent its newest air defence systems including the SA-22 into eastern Ukraine and is now directly involved in the fighting".

Francois Hollande, the French president, warned on Thursday it would be "intolerable and unacceptable" if Russian troops were on the ground in Ukraine.

The European security body the OSCE will hold a special meeting on Thursday to discuss developments in Ukraine following the reports that Russian troops were on the ground.