Ukraine's envoy to European security body the OSCE said on Thursday that regular Russian forces had seized the southeastern city of Novoazovsk, but his Russian counterpart said no Russian forces were crossing the border.
Ihor Prokopchuk, Ukraine's representative to the Organisation for Security and Cooperation in Europe, told reporters that "what we registered is a direct invasion of the Russian military into the eastern regions of Ukraine".
Speaking on the sidelines of an extraordinary OSCE meeting in Vienna, he added: "The city of Novoazovsk and a number of other cities close to this one have been seized by regular Russian forces".
Ukraine viewed the latest development as an "act of aggression", he said. "We will use all means available to Ukraine under international law, under U.N. charter, to defend its territory," Prokopchuk said.
In Kiev, Ukraine's security and defence council said the border town of Novoazovsk and other parts of Ukraine's south-east had fallen under the control of Russian forces who together with rebels were staging a counter-offensive.
However, Russian Ambassador Andrey Kelin separately told reporters during the meeting of the 57-nation OSCE: "No Russian forces are crossing in any point the border of Ukraine."
Apart from a small unit of about 10 Russian soldiers who he said had "unintentionally" crossed the border a couple of days ago, Kelin said that "there are no Russian forces over there".