US mulling 'serious sanctions' against Russia post ceasefire violations in east Ukraine

While accusing Russia of engaging in "an absolutely brazen and cynical process" over the last few days by supporting the rebels and undermining the ceasefire agreement, Kerry, who was a visit to London, warned that the U.S. was not going to "sit there and be part of this kind of extraordinarily craven behaviour at the expense of the sovereignty and integrity of a nation," reported the BBC.

He said that if this failure continued then there will be "further consequences" including, "consequences that would place additional strains on Russia's already troubled economy."

Kerry said that President Barack Obama would make a decision after weighing the options before him in the coming days.

However, a Kremlin spokesman remarked that sanctions would not help solve the Ukraine crisis.

The ceasefire that took effect last week was violated several times in the past few days.

However, the ceasefire deal appears to have been bolstered after a prisoner swap , one of the key points of the Minsk truce deal, was carried out in east Ukraine.

At least 139 Ukrainian troops and 52 rebels were freed.

More than 5,700 people have been killed in the conflict that began last April and some 1.5 million people have been displaced, according to the United Nations.

The Ukrainian government, Western nations and Nato say that there was clear evidence that Russia was providing military support to rebels in east Ukraine, a claim that has been denied by Moscow.