Government forces and pro-Russian separatists said they would not carry out an agreement to pull back heavy guns in east Ukraine on Tuesday, pushing a shaky peace deal closer to collapse.
Fighting has eased in many areas since a ceasefire came into force on Sunday, but the rebels have refused to halt attacks on a town where Ukrainian forces are encircled and Kiev says it will not pull back its big guns until the truce holds.
Monitors from the OSCE security group were expected to try to reach the besieged town, Debaltseve, after Germany said it had agreed steps with the leaders of Russia and Ukraine to ensure the observers had "free access" in the east.
But a call by Berlin for the withdrawal of heavy weaponry to start seemed to have fallen on deaf ears, though officials from Russia, Ukraine, the OSCE and the rebels were expected to discuss implementation of the peace deal by phone.
"We do not have the right (to stop fighting for Debaltseve). It's even a moral thing. It's internal territory," Denis Pushilin, a senior separatist representative, told Reuters in the rebel stronghold of Donetsk.
"We have to respond to fire, to work on destroying the enemy's fighting positions."
Asked about plans to carry out the agreement to withdraw big guns, he said: "We are ready at any time, we have everything ready for a mutual withdrawal. We will not do anything unilaterally. That would make our soldiers targets."
Ukraine's military reiterated that its forces could also not start withdrawing big weapons such as heavy artillery, as set out in the deal reached at marathon talks brokered by France and Germany in the Belarussian capital Minsk last Thursday.
"The pull-back can happen only on fulfillment of the first point of the Minsk agreements -- the ceasefire. In the last 24 hours there has been firing so there is no ceasefire and so there is no precondition for a pull-back of heavy weapons either," military spokesman Andriy Lysenko said.
Asked by Reuters whether the pull-back could start without a ceasefire at Debaltseve, a strategic railway junction, he said: "The Minsk agreements talk about a general ceasefire. So my answer is 'No, not possible'."
WESTERN CONCERN MOUNTS
Hopes that Thursday's deal will end a conflict that has killed more than 5,000 people have been dampened by the collapse of an earlier truce last month.
The European Union kept pressure on Russia and the rebels by announcing a new list of separatists and Russians targeted with sanctions on Monday. Moscow promised an "adequate" response.
The United States said it was "gravely concerned" by the fighting in and around Debaltseve and that it was closely monitoring reports of a new column of Russian military equipment moving toward the region.
"These aggressive actions and statements by the Russia-backed separatists threaten the most recent ceasefire," U.S. State Department spokeswoman Jen Psaki said in a statement.
"We call on Russia and the separatists it backs to halt all attacks immediately."
Russia denies sending arms or troops to back the rebels in mainly Russian-speaking areas of east Ukraine despite what Kiev and the West say is incontrovertible proof.
The separatists said soon after the ceasefire came into effect they had no intention of observing it at Debaltseve, where they have been advancing since January and now have a Ukrainian unit all but encircled.
The OSCE said on Sunday the rebels had refused to allow its monitors into Debaltseve after the truce took effect.
Fighting began in east Ukraine after the overthrow of a Moscow-backed president in Kiev a year ago and Russia's annexation of the Crimea peninsula a month later.
The West fears Putin, who has called parts of Ukraine "New Russia," wants the conflict to fester for years so that Kiev cannot control east Ukraine and is prevented from joining NATO, while Russia can retain influence there.
Moscow accuses the West of waging a proxy war in Ukraine to seek "regime change" in Moscow and "contain" Russia.
Western countries say they reserve the option of expanding economic sanctions on Moscow over the crisis, hoping a growing financial crisis in Russia will persuade Putin to use his influence with the rebels to stop the fighting.
Some analysts say Putin is banking on Ukraine's economy collapsing much faster than Russia's and on Russians remaining united behind him over the seizure of Crimea, which has boosted his popularity ratings at home.