Ukraine’s President said yesterday that Russian troops had entered his country in support of pro-Moscow rebels who captured a key coastal town, sharply escalating a five-month-old separatist war.

Petro Poroshenko told a meeting of security chiefs that the situation was “extraordinarily difficult ... but controllable” after Russian-backed rebels seized the town of Novoazovsk in the southeast of the former Soviet republic.

Earlier he said he had cancelled a visit to Turkey because of the “rapidly deteriorating situation” in the eastern Donetsk region, “as Russian troops have actually been brought into Ukraine”.

Russia’s Defence Ministry again denied the presence of its soldiers in Ukraine, using language redolent of the Cold War, even as two human rights advisers to President Vladimir Putin said more than 100 Russian troops had died there in a single attack on August 13.

Russia is now directly involved in the fighting

“We have noticed the launch of this informational ‘canard’ and are obliged to disappoint its overseas authors and their few apologists in Russia,” a defence ministry official, General-Major Igor Konashenkov, said . “The information contained in this material bears no relation to reality.”

But Western governments appeared to be running out of patience with Moscow’s denials.

Referring to talks that Putin held with Poroshenko just two days ago, British PM David Cameron said: “It is simply not enough to engage in talks in Minsk, while Russian tanks continue to roll over the border into Ukraine. Such activity must cease immediately.”

“We assess well over 1,000 Russian troops are now operating inside Ukraine,” said Dutch Brigadier-General Nico Tak, head of Nato’s crisis management centre. “They are supporting separatists [and] fighting with them.”

Rebel advances this week opened a new front in the conflict just as Ukraine’s army appeared to have gained the upper hand, virtually encircling the separatists in their main strongholds of Donetsk and Luhansk.

Ukraine’s security and defence council said Novoazovsk and other parts of southeast Ukraine had fallen under the control of Russian forces, and a counter-offensive by Russian troops and separatist units was continuing.

It said Ukrainian government forces had withdrawn from Novoazovsk “to save their lives” and were now reinforcing defences in the port of Mariupol further west, which a rebel leader said was the separatists’ next objective. Despite Russia’s denials, a member of Putin’s advisory council on human rights, Ella Polyakova, told Reuters she believed Russia was carrying out an invasion of 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, cross the border, I consider this an invasion,” Polyakova said.

The US ambassador to Kiev, Geoffrey Pyatt, tweeted: “Russian supplied tanks, armored vehicles, artillery and multiple rocket launchers have been insufficient to defeat Ukraine’ armed forces. So now an increasing number of Russian troops are intervening directly in fighting on Ukrainian territory.”

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