US Secretary of State John Kerry accused pro-Russian separatists on Wednesday (January 21) of attempting a "blatant land grab" during fresh fighting in eastern Ukraine.

Speaking at a news conference with the European Union foreign policy chief Federica Mogherini, Kerry said that the US was "particularly concerned" by a rebel initiative to "attempt to gain control of a very significant rail juncture" in eastern ukraine. He said that was in violation of a September ceasefire deal, calling it an "effort to try to broaden the amount of territory that is being held."

Kerry said there had been about a 500 square mile extension of the line of control which demarcates the rebel-held territory from the rest of Ukraine.

"So this is a blatant land grab and is in direct contravention to the Minsk agreements which they signed up to," Kerry added.

The US and EU have together imposed sanctions on Russia, which have contributed to an downturn in the Russian economy.

The EU sanctions are due to be reviewed in March after one year in force, but Mogherini stressed that "any kind of decision on sanctions is going to be based only on the full implementation of the Minsk agreement."

"It is in the hands of the Russians now to fulfill their own commitments... and that is going to be the only basis on which the European Union is going to decide any further steps on sanctions," she said.

She urged Moscow to fulfill all the commitments it had undertaken in signing the accords in the Belarus capital last year.