LONDON — The U.S. and Britain on Saturday discussed new sanctions against Russia as Ukraine’s week-old cease-fire unravels, though it was unclear if the allies were on the same page about increasing pressure on an economy that means far more to Europe.

As Ukraine’s military and Russia-backed separatists blamed each other for the continued attacks, U.S. Secretary of State John Kerry and British Foreign Secretary Philip Hammond held Moscow responsible. Kerry accused the Russians of “land-grabbing” in Ukraine while cynically speaking of peace.

The top American diplomat pointed specifically to the strategic port of Mariupol, which Ukrainian forces say is threatened by a buildup of rebel military equipment. If the rebels seize the city, they would establish a land corridor between mainland Russia and the Russia-annexed Crimean Peninsula.

“What’s happening with respect to Mariupol even now is just simply unacceptable, so we are talking about additional sanctions, additional efforts,” Kerry told reporters.

The message to Russian President Vladimir Putin’s government, 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 behavior at the expense of the sovereignty and integrity of a nation.”

Ukrainian forces on Saturday reported attacks over the past day that killed a serviceman and wounded 40, with mortars reaching the fringes of Mariupol, among other places. The rebels said Ukrainian forces shelled 15 locations overnight, including parts of Donetsk, the largest separatist-controlled city.

An agreement reached by the leaders of Ukraine, Russia, Germany and France last week called for the guns to go quiet Sunday, followed by a retreat of heavy weaponry from the front lines. International monitors have reported no such activity.

The U.S. has warily backed the diplomatic effort despite little faith in the Kremlin changing course. Washington repeatedly has invoked the threat of new sanctions if Russia doesn’t cut off the separatists.

Economic measures against Russia in the past year has severely damaged the country’s economy, but done little to change the calculus of Putin’s government in Ukraine.

It’s unclear whether the U.S. has sufficient support among its partners in Europe for a new round of trade or financial restrictions. Europe’s participation is seen as necessary, given its far deeper economic relationships with Russia.

Hammond criticized what he termed Russia’s “continued aggression” and systematic violations of the cease-fire agreement. But he did not explicitly mention sanctions.

“We will talk about how we maintain European Union unity and U.S.-European alignment in response to those breaches,” Hammond said.