Ukraine 's president has confirmed that Russian troops have entered his country over the last few days and are actively supporting pro-Russian rebels fighting government troops in the east.

President Petro Poroshenko described the situation as "extremely difficult but controllable," as he met with senior military officers while Russian-backed rebels were attacking the town of Novoazovsk in the southeast.

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Later in the day, Poroshenko received a report that the town had fallen and is now under rebel control.

Earlier, Poroshenko decided to postpone a trip to Turkey because of the rapidly deteriorating situation in Ukraine, saying that Russian troops had actually been brought into Ukraine.

Moscow continues to deny it has sent troops across the border. Russia's defense ministry said told Interfax news agency that "the information... bears no relation to reality."

But NATO says Russia has significantly escalated its "military interference" in Ukraine over the past few weeks.

Dutch Brigadier-General Nico Tak said at NATO's military headquarters in Brussels that battalions of Russian soldiers are now inside Ukraine.

"We assess well over 1,000 Russian troops are now operating inside Ukraine," said General Nico Tak, who heads NATO's crisis management centre. "They are supporting separatists and fighting with them."

Tak showed reporters a satellite picture, taken on Aug. 23, showing Russian self-propelled artillery lined up inside eastern Ukraine. He said these are highly sophisticated equipment, and that it is extremely unlikely these sorts of units are manned by separatists.

A separatist leader who describes himself as the prime minister of the Donetsk People's Republic added more details about Russian military involvement Thursday, saying about 3,000 Russian volunteers are serving in the rebel ranks, particularly the group that is now attacking Mariupol, a sea port town in the east that could open a new avenue for the entry of Russian forces by the sea.

Sources in Moscow also tell of a large column of Russian armor and troops that encountered Ukrainian military forces on Aug. 13 in the city of  Snizhnye in Donetsk province, and suffered 100 dead when Ukrainian tank and rocket fire ignited ordnance carried by Russian trucks that were also carrying soldiers.

Many of the wounded soldiers who came back told stories about how badly their column was hit well inside Ukraine territory.

A member of President Vladimir Putin's advisory council on human rights, Ella Polyakova, told Reuters news agency that she believed Russia has started an invasion into Ukraine.

"When masses of people, under commanders' orders, on tanks, APCs and with the use of heavy weapons, are on the territory of another country, I consider this an invasion," Polyakova said.

TagsUkraine, Russia