A two-day old truce in eastern Ukraine is on the verge of collapsing as fighting rages around a strategic railway hub.
The United States has called on Russia and the separatists it backs to "immediately" halt attacks in eastern Ukraine, expressing grave concern about ceasefire violations.
A two-day old truce was on the verge of collapsing as fighting raged around a strategic railway hub and both sides said conditions were not met to start withdrawing heavy weapons from the frontline.
The worst of the violence was around the town of Debaltseve, a key city between the separatist-held cities of Donetsk and Lugansk, where thousands of government troops were mostly surrounded by heavily armed rebels.
"The United States is gravely concerned by the deteriorating situation in and around Debaltseve in eastern Ukraine," State Department spokeswoman Jen Psaki said in a statement.
"We call on Russia and the separatists it backs to halt all attacks immediately, engage with the OSCE (Organisation for Security and Co-operation in Europe) to facilitate the ceasefire, and... fully implement their September 5 and 19 Minsk commitments."
Kiev officials and rebels accused each other of ongoing attacks that prevented them from pulling back tanks, rockets and artillery from the frontline in Ukraine's east.
An OSCE team tasked with monitoring the ceasefire has been unable to enter Debaltseve because of the hostilities.
Ukraine's government said the rebels had launched more than 112 assaults on its troops' positions on Sunday, killing five soldiers and wounding 25.
Monday saw another 38 attacks, most of them by Grad multiple rocket launchers and mortars, the defence ministry said on its Facebook page.
It claimed the separatists themselves had fired on Donetsk to blame the attack on government forces.
"These aggressive actions and statements by the Russia-backed separatists threaten the most recent ceasefire and jeopardise the planned withdrawal of heavy weapons, as called for in the February 12 Minsk packet of implementing measures," Psaki said.
She noted that the United States was "closely monitoring" reports of a new column of Russian military equipment making its way toward Debaltseve.