An armed pro-Russian separatist looks back next to a vehicle of the Organisation for Security and Cooperation in Europe's (OSCE) monitoring mission in Ukraine.
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 at "any point."
Ihor Prokopchuk, Ukraine's representative to the Organization 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 the UN charter, to defend its territory, to defend its independence."
In Kiev, Ukraine's security and defense council said Novoazovsk, a border town, and other parts of Ukraine's south-east had fallen under the control of Russian forces who together with rebels were staging a counteroffensive.
At the meeting of the OSCE, which groups Russia as well as the United States among its 57 members, Prokopchuk said he had appealed "to all our partners to provide all possible support to Ukraine to defend itself and to rebuff this aggression."
Russia denies intervening in Ukraine by arming the rebels or sending soldiers across the border. The defense ministry declined to comment on reports of Russian tanks in Novoazovsk.
Russian Ambassador Andrei Kelin told reporters during the OSCE meeting: "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."
Kelin said there was a worsening humanitarian situation in eastern Ukrainian regions where "people continue to die" because of what he said was shelling and artillery fire by the Ukrainian army on the population.
Rebel advances this week have opened a new front in the conflict just as Ukraine's army appeared to have gained the upper hand, virtually encircling the separatists in their main strongholds of Donetsk and Luhansk.
The US envoy to the OSCE, Daniel Baer, said that in the last 24 hours there had been "mounting reports of large numbers of Russian military personnel inside Ukraine, columns of Russian tanks and armored vehicles crossing into southeastern Ukraine."
"These indicate that a new Russian escalation, including a Russia-directed counteroffensive, may be underway in Donetsk and Luhansk oblasts — actions that are in clear violation of international law and OSCE principles and commitments," he said.