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Britain and the US have discussed further sanctions against Russia in the wake of the breakdown of a ceasefire in Ukraine.
US Secretary of State John Kerry said Moscow is being "extraordinarily craven" in its support for Ukrainian separatists in fighting that has raged in recent days.
Mr Kerry said before a meeting in London with British Foreign Secretary Philip Hammond that the two countries would discuss new economic measures to put pressure on Russia to end such support for the rebels.
He said: "Russia has engaged in an absolutely brazen and cynical process over these last days.
"We know to a certainty what Russia has been providing to the separatists, how Russia is involved with the separatists.
"This is behaviour that is completely counter to everything that the global community has worked to achieve since World War II."
Mr Kerry accused Moscow of "land-grabbing" in Ukraine while cynically talking about peace.
He pointed specifically to the port of Mariupol, which pro-government forces say is threatened by a build-up of rebel military equipment.
If the rebels take the city, they would establish a land corridor between mainland Russia and the Russian-annexed Crimean Peninsula.
Mr Kerry said: "What's happening with respect to Mariupol even now is just simply unacceptable, so we are talking about additional sanctions, additional efforts."
The message to Russian President Vladimir Putin, he said, would be: "We're not going to play this game.
"We're 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."
Mr Hammond also condemned the "unacceptable" way in which the ceasefire had been "systematically breached".
It comes as former Defence Secretary Liam Fox said the West should supply sophisticated weaponry, including anti-tank weapons and drones.
Kiev and the rebels have both accused each other of keeping up the attacks.
Ukrainian forces on Saturday reported attacks over the past day that killed one serviceman and wounded another 40, with mortars hitting the fringes of Mariupol, among other locations.
The rebels said Ukrainian forces shelled 15 locations overnight, including parts of Donetsk, the largest city the rebels control.
The current crisis was sparked by the ousting of Moscow-leaning president Viktor Yanukovych last February.
Weeks after Mr Yanukovych's removal, Moscow annexed the Crimean Peninsula and pro-Russian separatists seized key buildings in Ukraine's east, sparking a conflict that has killed more than 5,000 people.
The country's current president, Petro Poroshenko, has accused an influential aide to Mr Putin of being behind the killings of 100 activists during the demonstrations.
In Moscow, thousands of protesters gathered to demonstrate against what they called a "fascist coup" in Ukraine.