The Obama administration is weighing a new round of sanctions against Russia in response to its continued "land-grabbing" in easternUkraine despitea cease-fire agreement, Secretary of State John Kerry said Saturday.

Speaking to reporters in London, where he met with British Foreign Secretary Philip Hammond, Kerry said he expected that the United States and its European allies would impose some "very serious" sanctions and other steps to punish Moscow after repeated cease-fire violations by Russian-backed Ukrainian separatists.

Kerry said one of the most egregious violations was aseparatistassault on the strategically important railway hub of Debaltseve last week. President Barack Obama and European leaders have vowed there will be consequences for violations of a Feb. 12 peace agreement. Amid ongoing violence and chaos in eastern Ukraine, their credibility could be undermined if those tough words are not backed up by action.

"If this failure continues, make no mistake," Kerry warned. "There will be further consequences, including consequences that would place added strains on Russia's already troubled economy. We're not going to sit back and allow this kind of cynical, craven behavior to continue at the expense of the sovereignty and integrity of another nation.

"And I am confident that the United States and the United Kingdom and others are prepared to stand up and take the measures necessary to add to the cost of these actions."

Kerry said satellite imagery proves that Moscow has kept up its support of the rebel fighters despite signing the agreement brokered in Minsk by German Chancellor Angela Merkel and French President François Hollande. The pact called for both sides to pull back their heavy weaponry. That has not happened, Kerry said.

"We know to a certainty what Russia has been providing, and no amount of propaganda is capable of hiding these actions," Kerry said.

However, Kerry said Obama has not decided whether to send lethal defensive weapons to Ukraine, as many members of Congress have urged. He suggested the decision may depend on what happens in eastern Ukraine over the next several days.

Hammond also criticized Russia's "continued aggression" and said the Minsk agreement had been "systemically breached." But he did not predict more sanctions.

Europe is far more connected to the Russian economy than is the United States, and some countries in Europe are balking at further estrangement from Moscow, an important trading partner and a key supplier of natural gas for heat and power.

"We will talk about how we maintain European unity and the U.S.-European alignment in response to those breaches," Hammond said before he and Kerry sat down for talks.

In Ukraine, pieces of the cease-fire deal moved forward even as both sides said fighting was continuing. Pro-Russian rebels and the Ukrainian military swapped prisoners on Saturday, a key part of the peace plan reached after marathon negotiations this month.

Ukrainian President Petro Poroshenko said 139 Ukrainian prisoners of war had been released. One more was expected to be released Sunday, his spokesman said. Rebel officials said 52 people had been released to their side. Reporters on the scene described how wounded soldiers, some of them limping on crutches, walked miles to reach safety.

Poroshenkosaid many of the released Ukrainian soldiers were captured in Debaltseve, the site of a major defeat for government forces. Thousands of encircled troops fled in the dead of night over the frozen steppe, taking fire from two sides. The violence has threatened to upend the peace deal and has damaged Poroshenko politically.

Rebels and the Ukrainian military said Saturday that fighting had largelyquieted butthat some violence persisted in limited pockets of a region that before the war was Ukraine's industrial heartland. A Ukrainian military spokesman, Col. Andriy Lysenko, said one soldier had been killed and 40 wounded in the previous 24 hours.

Lysenko said that much of the fighting took place in a village just east of the vital Ukrainian-held port city of Mariupol. Rebels have moved heavy weaponry toward the city, he said, amid fears of an attack. The economy in the breakaway part of Ukraine would be far stronger if rebels possessed the port, and Mariupol could also form a strategic pro-Russian belt to connecttothe Russian-held Crimean Peninsula.

Poroshenkohas pleaded for an international peacekeeping mission, a tacit admission that Ukraine's beleaguered military cannot defend the country on its own.

In Moscow, thousands of protesters rallied in a central square against Ukraine and the West on the one-year anniversary of the ouster of Ukrainian President Viktor Yanukovych. The march was a measure of the powerful anti-Western sentiments that have grown here over the past 12 months. Tensions between the West and Russia are at their worst since the Cold War.

"Obama kills Donbas children," one poster read, referring to the eastern Ukrainian region where rebels have seized power.

A year ago, Russia was riding high as the Winter Olympics in Sochi came to a close. Now 74 percent of Russians have a negative view of the United States, according to polls taken by the independent Moscow-based Levada Center, up from 44 percent in January 2014.

Ukraine, too, has radically reshaped itself, electing the most pro-Western leaders in its history.

Yanukovych, the former president, gave a blitz of interviews on Russian television stations Saturday after staying out of the spotlight for most of the year. The ex-leader, who has lived in exile near Moscow, appeared heavier and wearier than he did a year ago. He said he hoped to return to Ukraine to lead a protest movement against the current government.