Separatists, backed by Russian soldiers, have captured the town of Novoazovsk in south-eastern Ukraine, agency reports said on Thursday
"There is military equipment in Novoazovsk which came across the border two days ago from Russia," a pro-Ukrainian government militia fighter 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, he said.
The BBC meanwhile quoted a pro-Russian rebel leader in eastern Ukraine has saying that 3-4,000 Russian citizens were fighting in their ranks.
Alexander Zakharchenko told Russian TV many of the Russians were former service people or current personnel on leave, the BBC reported.
Russia 'directly involved': US ambassador
"An increasing number of Russian troops are intervening directly in fighting in Ukrainian territory," Geoffrey Pyatt wrote on Twitter, adding that Moscow was "directly involved in the fighting" and had sent in its "newest air defense systems including the SA-22
European security body to meet over 'Russian involvement'
The European security body OSCE will hold a special meeting to discuss developments in conflict-torn Ukraine, following reports that Russian troops were on the ground.
"Russia violations in Ukraine to top agenda at extraordinary meet of OSCE delegations today in Vienna," the US mission to the organisation said in a tweet.
The Organisation for Security and Cooperation in Europe confirmed the special session, which "will focus on latest developments in Ukraine".
(With inputs from Reuters)