Ukrainian President Petro Poroshenko has convened an emergency meeting of the National Security and Defense Council after accusing Russia of sending troops into Ukraine.
Poroshenko canceled a planned trip to Turkey on August 28 and said he will meet with top officials to discuss the "sharp deterioration" of the situation in the Donetsk region, including the town of Novoazovsk, which the council says has fallen to Russian troops.
"The president's place is in Kyiv today," Poroshenko said.
He said he is also calling for extraordinary sessions of the UN Security Council and the European Council to discuss the crisis.
Western news agencies quoted a NATO military officer as saying the alliance believed "more than 1,000 Russian troops" are operating inside Ukraine.
"They are supporting separatists [and] fighting with them," the officer, who wished to remain anonymous, told reporters in Mons, Belgium.
He said it is conceivable that Moscow could create a land bridge to the Crimea Peninsula, which it annexed in March, via the southern Ukrainian city of Mariupol.
WATCH: RFE/RL's Andrei Babitsky shot footage of fierce fighting on the streets of the eastern Ukrainian city of Donetsk on August 28. (WARNING: Graphic content, including dead bodies)
The Organization for Security and Cooperation in Europe (OSCE) called a special session for August 28 to discuss the situation.
Ukraine's OSCE envoy, Ihor Prokopchuk, accused Russia of launching a "direct invasion," while Russia's OSCE envoy, Andrei Kelin, denied that Russian forces had crossed the Ukrainian border "at any point."
Maja Kocijancic, a spokeswoman for EU foreign-policy chief Catherine Ashton, told reporters on August 28 that the EU was "extremely concerned by the latest developments, including reports on what is happening on the ground."
The National Security and Defense Council said on Twitter on August 28 that Russian troops had seized control of Novoazovsk, on the Sea of Azov about 10 kilometers from the Russian border, and several settlements in the Novoazovsk, Starobeshevskiy, and Amvrosiyivskiy districts. Kyiv has accused Moscow of opening a second front in the region.
"A counteroffensive by Russian troops and separatist units is continuing in southeast Ukraine," the council said in a tweet.
The council said Ukrainian troops near Novoazovsk came under attack by Grad missile systems fired from Russian territory before being attacked by two convoys of Russian armor in Ukraine.
West Pressures Russia To Explain
Western officials have increased their pressure on Moscow in recent days, directly accusing the Russian military of directly intervening in eastern Ukraine.
Moscow has long denied arming the rebels or sending troops into Ukraine.
In a tweet on August 28, the U.S. ambassador to Ukraine, Geoffrey Pyatt, said "an increasing number of Russian troops are intervening directly in fighting on Ukrainian territory."
He said Russian-supplied tanks, armored vehicles, artillery, and rocket launchers "have been insufficient to defeat Ukraine's armed forces."
He also used Twitter to say Russia was sending "its newest air-defense systems including the SA-22 into eastern Ukraine & is now directly involved in the fighting."
In a phone call with President Vladimir Putin on August 27, German Chancellor Angela Merkel said reports of a Russian military incursion "must be explained" and emphasized Moscow's "major responsibility for de-escalation."
The U.S. State Department said on August 27 it suspected a "Russian-directed counteroffensive" was under way in eastern Ukraine.
State Department spokeswoman Jen Psaki told reporters that the reports include descriptions of "columns of Russian tanks, multiple rocket launchers, and armored vehicles pushing toward communities in southeastern Ukraine."
Psaki also chided the Russian government for its "unwillingness to tell the truth," even as its soldiers are found inside Ukraine.
On August 28, French President Francois Hollande said it would be "intolerable and unacceptable" if Russian troops were operating on Ukrainian territory.
Speaking in Paris to French ambassadors, Hollande said, "Russia cannot hope to be a 21st-century power yet not respect the rules."
'Flat-Out, Overt, And Obvious'
Moscow denies sending troops into Ukraine, claiming Russian paratroopers captured by Ukrainian forces in recent days had "inadvertently" crossed the border.
In Warsaw, Polish Prime Minister Donald Tusk told parliament that "no one can take seriously" Russia's claims that it's not supporting the pro-Russian separatists in Ukraine.
And a senior NATO diplomat, speaking to reporters in Brussels on condition of anonymity, said Russia's support for the separatists "appears increasingly to be flat-out, overt, and obvious."
Kremlin spokesman Dmitry Peskov said claims about an upsurge in Russian military activity on the border with Ukraine had "nothing to do with reality."
He also said reports of secret funerals being held for Russian paratroopers killed in fighting in Ukraine should be "checked in great detail," but he could not confirm such checks had begun.
Independent media reports in Russia have spoken of the secret funerals of two paratroopers in northwestern Russia. Mourners are quoted as saying the men were killed in Ukraine.
The U.S. State Department said it had also taken note of reports of wounded Russian soldiers being treated in a St. Petersburg hospital and of Russian soldiers returning home from Ukraine to Russia for burial.
More than 2,000 people have died since April in fighting between Ukrainian forces and pro-Russian separatists in the eastern regions of Donetsk and Luhansk, according to figures compiled by the United Nations.
With reporting by Reuters, AP, AFP, dpa, ITAR-TASS, Interfax, and RFE/RL Brussels correspondent Rikard Jozwiak