Latvia's foreign minister Edgars Rinkevics has called on the United Nations to 'act accordingly' after Ukraine says Russia has sent troops in to its country.
The ministers comments come after Ukraine President Petro Poroshenko said Russia had deployed troops in the east of the country.
News reports say pro-Russia rebels took the coast town of Novoazovsk and threatened the strategic port city of Mariupol, in the region's south.
"Russian invasion in Ukraine must be considered by the UN Security Council as act of aggression, UN must react accordingly, this is war" said Rinkevics, posting on his twitter account.
But Russia says none of its military convoys had crossed the border.
Russian representative to the OSCE Andrey Kelin said:“We have said that no Russian involvement has been spotted, there are no soldiers or equipment.”
“Accusations relating to convoys of armored personnel carriers have been heard during the past week and the week before that,” he said. “All of them were proven false back then, and is being proven false again now.”
“There is no sense in these accusations,” he said, quoted by RT.
The Organization for Security and Co-operation in Europe (OSCE) is holding a special meeting in Vienna to discuss developments in Ukraine, the BBC reports.
The rebel successes constitute the opening of a new front in the conflict.
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.
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.
(Edited by Rayyan Sabet-Parry)