A UN-backed ceasefire showed signs of finally taking hold in Ukraine, even as the United States accused Russian President Vladimir Putin of destabilising the country through land grabs.
Wednesday was the first day since the truce took effect 10 days ago that no deaths were reported on the front, but the relative peace was met by fresh diplomatic sparring between Washington and Moscow.
Addressing US lawmakers, Secretary of State John Kerry said Russia and pro-Moscow rebels had failed to meet the terms of the ceasefire.
Putin had put in place policies that "violate all the international norms with respect to territory and behaviour," Kerry told the House foreign affairs committee.
"He has empowered, encouraged, and facilitated directly land grabs in order to try to destabilise Ukraine itself."
"To date, neither Russia nor the forces it is supporting have come close to complying with their commitments," he said, renewing warnings that Moscow would face further sanctions.
Top US officials have lashed Putin and his ministers in recent days, with Kerry on Tuesday directly accusing Russian leaders of lying "to my face" over the conflict.
Washington has repeatedly rejected Moscow's denials about giving military backing to the separatists in east Ukraine.
Russia has in turn warned it could cut off gas supplies to Ukraine within days -- and, by extension, to parts of the European Union.
- 'How dumb do I look? -
National Security Advisor Susan Rice echoed Kerry's concerns in an interview with PBS television.
Asked if she believed Putin's assertions that he wanted peace in Ukraine, the former US envoy to the United Nations said: "How dumb do I look?"
"One cannot accept Vladimir Putin at his word because his actions have belied his words repeatedly, particularly in the context of Ukraine."
NATO's top commander for Europe, Philip Breedlove, meanwhile accused Putin of making significant deployments of heavy weaponry in eastern Ukraine.
"Over a thousand combat vehicles, Russian combat forces, some of their most sophisticated air defence, battalions of artillery, Mr Putin has already set the bar... very high," he said.
The tensions between the old Cold War foes overshadowed a lull in fighting that produced the first day without fatalities since the ceasefire came into effect on February 15.
"Over the past day, one soldier was wounded but there were no dead," Ukrainian military spokesman Andriy Lysenko told journalists in Kiev. The rebels also reported no deaths on their side.
There was still no confirmation, though, from the Organization for Security and Cooperation in Europe (OSCE) of a pull-back of heavy weapons from the frontline -- the other key plank of the truce.
Rebels insisted they were withdrawing artillery, rocket launchers and tanks from some areas, and journalists saw a column of howitzer guns being driven along a road near the separatist stronghold of Donetsk.
But the OSCE mission in Ukraine said the warring sides had not provided the information needed to determine what, if any, arms withdrawals have occurred.
Kiev says it will not carry out an arms pull-back until a full and "comprehensive" ceasefire is observed and has accused Russia of continuing to send military hardware in to bolster the rebels.
- Exasperation with Russia -
The West has thrown its hopes of finding a negotiated solution to the 10-month conflict fully behind the truce, which last week won unanimous backing from the UN Security Council.
But continued breaches by rebel forces -- especially their assault on Debaltseve, a strategic transport hub, and attacks on Ukrainian army positions near the port city of Mariupol -- have exasperated the EU and US.
British Prime Minister David Cameron this week announced his country will send up to 75 soldiers to Ukraine on a "training mission". He said they would not be sent to the conflict zone.
Poland on Wednesday announced its own plans to send a small contingent of troops to Ukraine to help train Kiev's military officers.
Cameron urged the EU to look at wide-ranging sanctions on Russia's economy, which is already toppling into recession because of a drop in oil prices.
Moscow has flexed its muscles in readiness for any further sanctions. Its state-run gas giant Gazprom has warned it could cut off supplies to Ukraine, amid a dispute over payment.
Putin said Wednesday that Ukraine had paid only enough "for three or four days' gas supplies. Unless there is a prepayment, Gazprom... will terminate the supply", he said, according to the Interfax news agency.
Much of the gas that flows through Ukraine goes on to supply the EU market.
Putin, in televised comments, also claimed Ukraine had cut off gas supplies to separatist-held territories in the east in what "smacks of genocide."
The instability is damaging Ukraine's fragile economy. The country's central bank on Wednesday increased currency controls in a bid to stop a free-fall of its currency, the hryvnia.