Our Brussels correspondent has quoted a NATO military officer as saying the alliance believes "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.
From our news desk:
Western news agencies quote a NATO military officer as saying the alliance believes "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.
The announcement came hours after Ukrainian President Petro Poroshenko accused Moscow of sending troops into Ukraine.
Poroshenko convened an emergency session of the country's National Security and Defense Council 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.
Moscow has long denied arming the rebels or sending troops into Ukraine.
At a meeting in Vienna on August 28, Ukraine's OSCE envoy, Ihor Prokopchuk, accused Russia of launching a "direct invasion," while Russia's envoy, Andrei Kelin, denied Russian forces had crossed the Ukrainian border "at any point."
A sentiment that is getting quite hard to disagree with.
BREAKING: Western news agencies quote a NATO military officer as saying the alliance believes 'more than 1,000 Russian troops' are operating inside Ukraine. (Reuters, AFP)
Further clarification from our news desk on Poroshenko's statement:
Ukrainian President Petro Poroshenko has convened an emergency meeting of the country's 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.
He said he is also calling for extraordinary sessions of the UN Security Council and the European Council to discuss the crisis.
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.
Western officials have accused the Russian military of directly intervening in eastern Ukraine.
Moscow has long denied arming the rebels or sending troops into Ukraine.
We're trying to ascertain what Poroshenko's original language was, but Reuters just issued this correction.
CORRECTED-Ukraine's Poroshenko says Russian troops have been brought into Ukraine
(Corrects to show statement on presidential website refers to Russian troops having been "brought into Ukraine", not an "invasion")
KIEV, Aug 28 (Reuters) - Ukrainian President Poroshenko said on Thursday Russian forces had been "brought into Ukraine" and he called an urgent meeting of Ukraine's security and defence council to decide the next steps to take in the crisis.
"I made the decision to cancel a working visit to the Republic of Turkey in connection with the rapidly deteriorating situation in Donetsk region, in particular in Amvrosiyivka and Starobesheve, as Russian troops have actually been brought into Ukraine," he said in a statement on the presidential website.
(Reporting by Pavel Polityuk; Writing by Alessandra Prentice; Editing by Richard Balmforth and John Stonestreet)
Poroshenko's video statement.