The United States and Britain are discussing new sanctions against Russia as Ukraine's week-old cease-fire proves ineffective.

U.S. Secretary of State John Kerry says Moscow is being "extraordinarily craven" for supporting Ukrainian separatists in fighting that has raged on in recent days.

Kerry said before a Saturday meeting in London with British Foreign Secretary Philip Hammond that the U.S. and Britain would talk about new economic measures to pressure Russia to end such support for the rebels.

Hammond also criticized Russia's "breaches" of the cease-fire agreement, but he was less explicit about how his government might respond.

Ukraine's military and the rebels are accusing each other of keeping up the attacks.