Ukraine officials say the key border town of Novoazovsk in southeastern Ukraine has fallen under the control of Russia's military, and NATO reports of more than 1,000 Russian troops operating inside the country's borders have sparked fears of what Ukraine's ambassador to Europe's security organization calls "a direct invasion."
At a special Organization for Security and Cooperation meeting in Vienna to discuss developments in Ukraine, that country's OSCE ambassador, Ihor Prokopchuk, told reporters "what we registered is a direct invasion of the Russian military into the eastern regions of Ukraine," Reuters reported.
But Russian Ambassador Andrey Kelin, also at the OSCE meeting, denied the allegation.
"No Russian forces are crossing in any point the border of Ukraine," he told reporters, disregarding that 11 Russian soldiers had been captured inside Ukraine this week.
Moscow has repeatedly denied charges of directly intervening in the conflict in eastern Ukraine, in which more than 2,200 people have been killed since April.
Meanwhile, a senior NATO official said the organization had assessed that "well over 1,000 Russian troops are now operating inside Ukraine. ... They are supporting separatists [and] fighting with them." Reuters quoted the unnamed official, who was speaking from NATO's military headquarters in Belgium.
Ukraine's Security and Defense Council announced the Novazovsk news Thursday on Twitter, adding that several surrounding villages also have been seized.
The developments along Ukraine's southeastern front have raised concerns that Moscow wants to create a land link between Russia and Crimea, which Russia annexed in March.
Ukraine's President Petro Poroshenko cancelled a visit to Turkey and called an emergency meeting of his country's security and defense council – just two days after he and Russian President Vladimir Putin met in Belarus to work out a peace process.
Ukrainian president Petro Poroshenko makes a statement, at Boryspil airport in Kyiv, Aug. 28, 2014.
In a statement, Poroshenko said there was a "rapidly deteriorating situation" in the Donetsk region. He said Russian troops had "actually been brought into Ukraine."
The U.S. mission to the OSCE said on Twitter that "Russian violations" in Ukraine would top the Vienna meeting's agenda.
Geoffrey Pyatt, the U.S. ambassador to the Ukraine, said in a Twitter post Thursday that an "increasing number of Russian troops are intervening directly in fighting on Ukrainian territory."
The U.S. envoy to the OSCE, Daniel Baer, cited "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," Baer said, according to Reuters. The actions "are in clear violation of international law and OSCE principles and commitments.''
Ukraine's ambassador to the European Union on Thursday urged the country's Western allies to provide "large scale" military support to confront the growing threat, AFP reported.
And Ukraine's prime minister, Arseny Yatseniuk, has asked the United States and Europe "to freeze Russian assets and finances until Russia withdraws armed forces, equipment and agents," Reuters said.
On Wednesday, the U.S. State Department said Russia has sent more military assets into eastern Ukraine, "indicating a Russian-directed counteroffensive is likely underway" near two key rebel-held border cities.
Spokeswoman Jen Psaki cited reports of separatists shelling residential areas in the port city of Mariupol. She also said heavy fighting has erupted at the airport in the hub city of Donetsk.
The New York Times, reporting from southeastern Ukraine, said tanks, artillery and infantry have crossed into Ukraine from Russia at the border town of Novoazovsk in recent days.
For weeks, Western analysts have said Moscow has been aiding pro-Russian separatists who are trying to break the Ukrainian siege at rebel-held Luhansk. Officials also say separatists have been battling to open a supply corridor from the Ukraine-Russian border to nearby Donetsk.
Some information for this report was provided by Reuters, the Associated Press and Agence France-Presse.