Western leaders are seeking to save the ceasefire deal between the Ukrainian government and pro-Russian rebels as the rivals refuse to withdraw heavy weaponry from the established buffer zone as agreed.
The warring sides were scheduled to begin scaling back after midnight on Monday, but both of them said they were not going to do so.
Vladimir Kononov, the so-called defence minister of the rebel group the Donetsk People's Republic, said the separatists would start withdrawing their heavy weapons "synchronously with the Ukrainian government forces".
"The two sides should completely withdraw heavy weaponry within the 14 days starting from February 16," Kononov said. "However, judging from the current situation, the Ukrainian government forces have yet to start the process."
Earlier on Monday, a Ukrainian military spokesman, Vladyslav Seleznyov, told the AFP news agency that "there is no question at the moment of us withdrawing heavy weapons" from the front line, blaming the rebels of ignoring the truce.
Fighting subsided in many parts of eastern Ukraine under a ceasefire deal reached last week in talks involving the leaders of France, Germany, Russia and Ukraine.
But the truce appears to have been stillborn in Debaltseve, where the most intensive fighting has taken place in recent weeks.
The US called on Russia and the rebels to "immediately" halt attacks.
"The United States is gravely concerned by the deteriorating situation in and around Debaltseve in eastern Ukraine," Jen Psaki, the US state department spokeswoman, said in a statement.
The European Union pleaded that "the shooting needs to stop".
In Berlin, German Chancellor Angela Merkel appealed for the ceasefire to be respected. "The situation is fragile," she conceded. "[But] that was certainly to be expected with a view to Debaltseve."
Sawsan Chebli , the German foreign ministry spokeswoman, mentioned the possibility of a meeting between the German, French, Russian and Ukrainian foreign ministers this week to discuss "further steps" to implement the ceasefire deal.
Observers from the Organisation from Security and Co-operation in Europe (OSCE), who are supposed to monitor the ceasefire, said on Sunday that separatists denied them access to Debaltseve.
Source: Al Jazeera and agencies