In some of its sharpest criticism to date, the United States has accused Russia of "undermining" the global order by supporting rebels in eastern Ukraine.
Germany and France also demanded that a crumbling Ukraine truce be "fully respected," but the words appeared hollow as pro-Russian separatists celebrated a battlefield victory in the strategic town of Debaltseve.
"Russia's continued support of ongoing separatist attacks in violation of the ceasefire in eastern Ukraine is undermining international diplomacy and multilateral institutions - the foundations of our modern global order," US State Department spokeswoman Jen Psaki said.
She insisted that the ceasefire deals reached in Minsk in September and renewed last week were "the basis for a durable resolution in eastern Ukraine."
Ms Psaki said, "We call upon Russia to honor its commitments immediately with decisive action before we see more cities decimated and more lives lost in eastern Ukraine."
The European Union, US and Ukraine accuse Russia of being behind the hostilities, while Moscow denies directly supporting the rebels.
A fresh ceasefire was meant to go into force on Sunday, but Ukraine and the US have pointed to repeated violations, including the seizure of Debaltseve.
"By not abiding by the agreement they signed, by continuing to support and intervene illegally in Ukraine... they're violating international norms and they're violating international law," Ms Psaki said, referring to Russia.
She added that the US was in contact with the Organization for Security and Cooperation in Europe - the international body tasked with monitoring the Ukraine ceasefire - to see what extra help the US could provide.
"Our goal is to ensure that they are well equipped to carry out their task, including monitoring and implementation of the ceasefire, and monitoring the international border between Ukraine and Russia," Ms Psaki said.
The OSCE's mandate runs until March, and it currently has some 500 monitors in the country.
Earlier today, Ukraine's military said more than 20 Russian tanks, ten missile systems and bus-loads of fighters had crossed the border and was headed towards Novoazovsk, near Mariupol.
"In recent days, despite the Minsk (ceasefire) agreement, military equipment and ammunition has been tracked crossing from Russia into Ukraine," military spokesman Andriy Lysenko said
"More than 20 (tanks), ten self-propelled artillery systems and 15 trucks have entered in the direction of Novoazovsk," he said, adding that buses filled with fighters from Russia had also been spotted crossing the border in the area.
Fighting persisted in east Ukraine today despite new European efforts to ensure the ceasefire takes hold.
Ukraine said pro-Russian separatists had attacked positions held by government troops 49 times in the past 24 hours, using rockets, artillery and armoured vehicles.
A military spokesman said there had been some shelling in the district of Mariupol - mostly government-held territory on the Sea of Azov that Kiev fears may become the focus of the separatists' next offensive.
The rebels accused government forces of also staging attacks, including on what they said were residential areas of the rebel stronghold of Donetsk.
One woman was killed in the shelling yesterday the separatist press service DAN said.
The fiercest fighting since the truce came into effect on Sunday was in and around Debaltseve, a strategic rail hub, before Ukrainian forces withdrew on Wednesday.
The leaders of France, Germany, Ukraine and Russia agreed by telephone yesterday to make a new effort to ensure the ceasefire holds.
The four countries' foreign ministers are expected to meet next week to discuss east Ukraine.
EU 'guilty of sleepwalking' into Ukraine crisis
Meanwhile, a scathing report from a British parliamentary committee has said Britain and the European Union are guilty of "sleepwalking" into a crisis in Ukraine.
The criticism from the EU Committee of the House of Lords, scrutinises the British government's EU policies.
Kiev and the West accuse Russia of arming the insurgents and sending soldiers into Ukraine - something Russian President Vladimir Putin denies.
The committee "believes that the EU, and by implication the UK, was guilty of sleepwalking into this crisis," said committee chairman Christopher Tugendhat.
"The lack of robust analytical capacity, in both the UK and the EU, effectively led to a catastrophic misreading of the mood in the run-up to the crisis."
A report by the committee said the EU's relationship with Moscow had long assumed an "optimistic premise" that Russia was on the path to becoming more democratic.
It blamed a decline in expertise on Russia in Britain's foreign office and other EU ministries for an inability to give an "authoritative response".
"The government has not been as active or as visible on this issue as it could have been," the report said.
It is the latest blow to the administration of British Prime Minister David Cameron over the issue, after he was described as "a foreign policy irrelevance" in the crisis by Britain's top commander in NATO, General Richard Shirreff.
