'Russian troops deployed' in Ukraine - Poroshenko
28 August 2014 Last updated at 10:40 ET
Pro-Russian rebels have been fighting Ukrainian forces in the city of Donetsk for months
Ukrainian forces are being deployed to help defend the southern city of Mariupol
Shelling is continuing in Donetsk, hitting buildings including this school
Railway lines in eastern Ukraine have been destroyed in the fighting
'Russian troops deployed' in Ukraine - Poroshenko
Ukrainian officials have expressed a great deal of worry and concern, as David Stern reports
Ukraine's president has cancelled a visit to Turkey, citing "Russian troop deployments" in the country's east.
His remarks came as pro-Russian rebels opened a new front in the south by seizing the coastal town of Novoazovsk.
Nato says it has detected a significant increase of Russian arms being supplied to the rebels over the past two weeks.
Russia has denied that its forces have crossed Ukraine's border. At least 2,119 people have been killed in four months of fighting.
The United Nations Security Council is to hold an emergency meeting in New York on Thursday at 18:00 GMT to discuss the crisis.
Nato Brigadier General Niko Tak told the BBC that there had been a "significant escalation in the level and sophistication of Russia's military interference in Ukraine" over the past two weeks.
Pro-Russian rebels have been fighting Ukrainian forces in the city of Donetsk for months
"[Nato has] detected large quantities of advanced weapons, including air defence systems, artillery, tanks, and armoured personnel carriers being transferred to separatist forces in eastern Ukraine," he said.
"Russia is reinforcing and resupplying separatist forces in a blatant attempt to change the momentum of the fighting, which is currently favouring the Ukrainian military."
More than 1,000 Russian troops are reportedly operating inside Ukraine, both supporting the separatists and fighting on their side.
'Sharp aggravation'
On Thursday, President Petro Poroshenko said he had called a meeting of the Ukrainian security council in light of the deteriorating situation.
"I have made a decision to cancel my working visit to the Republic of Turkey due to the sharp aggravation of the situation in Donetsk region... as Russian troops were actually brought into Ukraine," Mr Poroshenko said in a statement.
Meanwhile Prime Minister Arseny Yatsenyuk said Russia had "unleashed a war in Europe", adding that the world should take "effective steps".
Ukrainian prime minister Arseniy Yatsenyuk: "Russian military boots are on Ukrainian ground'
But Russia's envoy at the Organisation for Cooperation and Security in Europe (OSCE), Andrey Kelin, denied there were any Russian troops.
Government forces had made significant advances against the separatists in recent weeks, but these gains seem in doubt with rebels now operating in two distinct areas of Donetsk region.
Separatist leader Alexander Zakharchenko told Russian TV that 3-4,000 Russian citizens were fighting in their ranks.
He said many of the Russians were former service-people or current service personnel on leave, insisting that all were volunteers.
In other developments:
- German Chancellor Angela Merkel demanded an explanation from Russia's President Vladimir Putin amid the reports of an incursion
- French President Francois Hollande said it would be "intolerable" if Russian troops were in Ukraine, and demanded that Russia stop sending aid to the rebels
- The OSCE is holding a special meeting in Vienna to discuss developments in Ukraine.
Ukrainian forces are being deployed to help defend the southern city of MariupolShelling is continuing in Donetsk, hitting buildings including this schoolRailway lines in eastern Ukraine have been destroyed in the fighting
Ukraine's security and defence council confirmed reports that Novoazovsk had been captured by the rebels, whom they described as "Russian troops".
It said it had withdrawn its forces to save lives, and that Ukrainian soldiers were now reinforcing the defences of the strategic port city of Mariupol.
The port has until now been peaceful and cut off from rebel positions.
A Ukrainian company commander, Vladimir Shilov, told Ukrainian TV that he had heard from sources inside the town that it was blocked by tanks and no-one was allowed to leave. Local officials had already fled to Mariupol, he added.
Francois Hollande: "If it were to be proven that Russian soldiers are on Ukrainian soil, this would be intolerable and unacceptable"
A spokesman for the rebels told Interfax news agency that Novoazovsk was under their control and they would soon "liberate" Mariupol.
Pro-Russian fighters have been trying for weeks to break out of an area further north in the Donetsk region where they are almost encircled.
Analysts say the separatists could also be seeking a land link between Russia and Crimea, which would give them control over the entire Sea of Azov.
Russia annexed the Black Sea peninsula of Crimea from Ukraine in March.bbc
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