Kiev: The United States on Thursday charged Russia with allowing its troops to "directly” intervene in fighting in the war-torn east of Ukraine.
US ambassador in Kiev Geoffrey Pyatt said that "an increasing number of Russian troops are intervening directly in fighting in Ukrainian territory".
Pyatt wrote on Twitter that Moscow was "directly involved in the fighting" and had sent in its "newest air defence systems including the SA-22".
Western intelligence agencies have said that Russian Army units were operating inside the country.
Meanwhile, rebels in east Ukraine appear to have seized swathes of territory from retreating government forces.
After weeks of government offensives that have seen troops push deep into the last rebel bastions, the tide appeared to be turning once again in the four-month conflict, prompting a nervous government in Kiev to call on NATO for help.
Journalists saw no signs of government troops south of the rebel-held city of Donetsk, with road blocks on the entire 100-kilometre (60-mile) stretch to the Azov Sea manned by pro-Russian rebels.
A volunteer commander posted on Facebook that government forces were surrounded in the key transport hub of Ilovaysk and reinforcements were desperately needed.
Evidence of the deteriorating situation emerged just hours after the first meeting in three months between Ukrainian President Petro Poroshenko and his Russian counterpart Vladimir Putin, which failed to achieve any concrete breakthrough.
There is increasing concern in Kiev and the West that Russia is intervening directly in the conflict -- a charge that Moscow has repeatedly denied.
Ukraine`s government claimed on Wednesday that a battalion of Russian soldiers had set up a military headquarters near the village of Pobeda, around 50 kilometres (30 miles) southeast of Donetsk.
(With agency inputs)