Germany and US voice concern on Russian troops in Ukraine

He added that Russian soldiers and armoured vehicles also crossed the border further north, near the Ukrainian town of Amrosievka.

Steffen Seibert, Chancellor Angela Merkel’s spokesman, said Merkel called Russian leader Vladimir Putin the same day to ask him if it is true.

“The latest reports of the presence of Russian soldiers on Ukrainian territory must be explained. She emphasised Russia's major responsibility for de-escalation and for watching over its own frontiers”, his statement said.

The Kremlin’s communique on the Merkel call said nothing on the subject, however.

Instead, it “announced the intention of the Russian side to provide new supplies of humanitarian aid to Luhansk and Donetsk” - two rebel-held areas where Russian "aid" trucks recently came and went without approval from Ukraine or the Red Cross.

The US was more explicit.

Jen Psaki, a State Department spokeswoman, told media in Washington on Wednesday that “these [new] incursions indicate a Russian-directed counteroffensive is likely underway in Donetsk and Luhansk”.

She added: “Russia is sending its young men into Ukraine … We also note reports of wounded Russian soldiers in a St. Petersburg hospital and that other Russian soldiers are returning home to Russia for burial”.

Geoffrey Pyatt, the US ambassador to Ukraine, said on Thursday morning: “An increasing number of Russian troops are intervening directly in fighting on Ukrainian territory. Russia … is now directly involved in the fighting”.

Wednesday’s developments come after Russia and Ukraine agreed to revive peace talks on Tuesday and ahead of an EU summit on Friday.

They also come at a decisive moment in military terms.

Ukrainian forces have encircled pro-Russia rebels in Luhansk and Donetsk. But if they do not score a decisive victory before October, when bad weather begins to set in, the fighting will be frozen until spring, giving the pro-Russia side a chance to dig in.

The coming winter is also raising the stakes on the gas side of the conflict.

Russia has stopped deliveries to Ukraine in a price dispute while promising to keep pumping gas to EU clients via Ukraine's pipelines.

On Wednesday, Ukraine’s PM, Arseniy Yatseniuk, told a government meeting that “we [Ukraine] know of Russia's plans to block [gas] transit even to European Union countries this winter”.

But Russian energy minister Alexander Novak denied the allegations.

“We can qualify them only as absolutely baseless speculations aimed at confusing or deliberately misinforming of European consumers of Russian gas”, he said.