Secretary of State John Kerry said on Saturday the United States and its allies were not prepared to play games with Russia and are discussing additional sanctions against Moscow over its role in eastern Ukraine, undermining a European-brokered truce.

"Russia has engaged in an absolutely brazen and cynical process over these last days," Kerry said as he began a meeting with British counterpart Philip Hammond.

"We are talking about additional sanctions, about additional efforts, and I'm confident over the next days people will make it clear that we are not going to play this game ... and be part of this kind of extraordinarily craven behavior."

A bipartisan group of senators Friday called on Secretary of State John Kerry to tighten economic sanctions on Russia and provide defensive weapons to Ukraine after Russian-backed rebels breached this month's cease- fire agreement and seized the Ukrainian town of Debaltseve.

The senators, including No. 2 Senate Democrat Dick Durbin and Armed Services Committee Republican Chairman John McCain, told Kerry in a letter that "it is time to provide defensive weapons to Ukraine and to consider imposing additional sanctions and penalties that will increase the cost of Putin's actions."

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"We can only conclude that in the absence of greater Western support for Ukrainian forces, a Russian-imposed military outcome will continue to unfold in Ukraine," they said.

The Senate letter was signed by six Republicans and four Democrats.