A pro-Russia rebel leader has said serving Russian soldiers, on leave from their posts, are fighting Ukrainian troops alongside the country's separatists in eastern Ukraine, the website of a Russian state television channel has reported.
"Among us are fighting serving soldiers, who would rather take their vacation not on a beach but with us, among brothers, who are fighting for their freedom," said Zakharchenko in an interview posted on Thursday on Vesti.ru.
Meanwhile, a pro-government militia fighter said on Thursday that separatists backed by Russian soldiers had entered the town of Novoazovsk in southeastern Ukraine.
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 appeared to confirm 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," the fighter, from the so-called Azov battalion which supports the Ukrainian army, told the Reuters news agency.
"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.
Arseniy Yatseniuk, Ukraine's prime minister, appealed to the United States, European Union and G7 countries on Thursday to freeze Russian assets until Russian forces withdraw from Ukrainian territory.
Meanwhile, Washington on Thursday accused Moscow of being "directly involved" in fighting in Ukraine.
Geoffrey Pyatt, the US ambassador to Kiev, wrote on Twitter that "Russia has also sent its newest air defence systems including the SA-22 into eastern Ukraine and is now directly involved in the fighting".
Francois Hollande, the French president, warned on Thursday it would be "intolerable and unacceptable" if Russian troops were on the ground in Ukraine.
The European security body the OSCE will hold a special meeting on Thursday to discuss developments in Ukraine following the reports that Russian troops were on the ground.