Kiev: Ukraine on Thursday said "Russian troops" had seized control of a key southeastern border town where fierce clashes had been raging.
"Yesterday Russian soldiers took control of the town of Novoazovsk," Ukraine`s National Security and Defence Council wrote on Twitter, adding that a number of surrounding villages had also been seized.
A military source said the separatist forces had also taken Savur-Mohyla, a hill east of Donetsk which gives strategic command over large areas of the territory.
The sudden reverses for the Ukrainian military confirmed the arrival of Russian forces to support the separatists, who have in recent weeks been under pressure from government forces in their strongholds of Donetsk and Luhansk, the east`s two main cities.
"There is military equipment in Novoazovsk which came across the border two days ago from Russia," a fighter, from the so-called Azov battalion which supports the Ukrainian army, had earlier told Reuters by telephone. "The equipment is carrying the flags of the DNR (Donetsk People`s Republic) rebels but they are regular Russian forces," the fighter who did not wish to be identified by name, said.
The loss of Novoazovsk is a blow to government forces since it leaves vulnerable the big port city of Mariupol, further west along the coast.
Ukraine`s President Petro Poroshenko scrapped his visit to Turkey and convened a top security meeting in view of Russian troops seizing the border town in the east.
East Ukrainian pro-Russian separatist leader Alexander Zakharchenko, meanwhile, said that serving Russian soldiers, on leave from their posts, are fighting Ukrainian troops alongside the rebels.
Also today, the US ambassador to Kiev, Geoffrey Pyatt, said in tweet: "Russian supplied tanks, armoured 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.
"Russia has also sent its newest air defence systems including the SA-22 into eastern Ukraine & is now directly involved in the fighting," he said.
(With agency inputs)