Russia will face fresh sanctions from the EU if a deal to end the Ukraine war is not fully implemented, German Chancellor Angela Merkel has warned.
She said EU leaders had asked officials to prepare further sanctions in case an agreed ceasefire was not respected.
The leaders of Russia, Ukraine, Germany and France sealed a deal on Thursday after marathon talks in Belarus.
The ceasefire is due to begin in eastern Ukraine at midnight on Saturday but both sides remain sceptical.
Pro-Russian rebels have signed the agreement, which also includes weapon withdrawals and prisoner exchanges, but key issues remain to be settled.
Clashes between government forces and the rebels continued on Thursday and one Russian-backed commander said his forces would not stop fighting.
Thousands of people have died in almost a year of fighting in the region.
In another development, the World Bank said on Thursday it was ready to provide up to $2bn billion (£1.3bn) in financial assistance to Ukraine this year as part of an international package of support.
At the scene: James Reynolds, BBC News, Donetsk city
Tonight, the centre of this rebel heartland is quieter than before. The sound of artillery fire in the distance has become more sporadic but it has not entirely ended.
Donetsk remains a semi-deserted city. Many residents who remember the failure of last September's ceasefire agreement will spend the night in their basements and shelters.
One local told me he hoped the ceasefire would work but said he wanted "New Russia" to remain in the future. "There are only a handful of people who will want to be part of Ukraine," he said.
Rebel forces continue to hold their checkpoints and positions and see no immediate need to retreat. One fighter said he did not trust the other side to stop shooting. Others have told the BBC that they will carry on fighting in the name of their fellow soldiers who have been killed.
'A good morning'
Ukrainian President Petro Poroshenko said implementation of the agreement, reached by leaders in the Belarusian capital Minsk, would be difficult.
A sticking point is the disputed town of Debaltseve, a key government-held town besieged by rebels, where fighting is still going on.
Further talks will also be held on self-rule in parts of Donetsk and Luhansk separatist regions.
- Ceasefire to begin at 00:01 local time on 15 February (22:00 GMT 14 February)
- Heavy weapons to be withdrawn, beginning on 16 February and completed in two weeks
- All prisoners to be released; amnesty for those involved in fighting
- Withdrawal of all foreign troops and weapons from Ukrainian territory. Disarmament of all illegal groups
- Ukraine to allow resumption of normal life in rebel areas, by lifting restrictions
- Constitutional reform to enable decentralisation for rebel regions by the end of 2015
- Ukraine to control border with Russia if conditions met by the end of 2015
Speaking after the 16-hour talks ended, Mr Putin told Russian television: "It wasn't the best night for me, but it's a good morning."
German Chancellor Angela Merkel said it offered a "glimmer of hope", while French President Francois Hollande said "the coming hours will be decisive".
The White House welcomed the agreement as a "potentially significant step" but expressed concern over reports of continued fighting in eastern Ukraine.
"All the parties must show complete restraint in the run-up to the Sunday ceasefire, including an immediate halt to the Russian and separatist assault on Debaltseve and other Ukrainian towns," Mr Kerry said in a statement.
Both Mr Kerry and Ms Merkel said sanctions on Russia could be lifted - but only if the Minsk agreement was respected.
But the German leader warned: "We hold open the possibility, if these new agreements are not implemented, that we must take further measures."
EU President Donald Tusk echoed her comments, saying the bloc would "not hesitate to take the necessary steps" if the agreement failed.
Earlier, separatists gave the agreement a cautious welcome but Donetsk rebel leader Alexander Zakharchenko said Kiev would be to blame if the deal collapsed and warned that there would "be no meetings and no new agreements".
Mr Poroshenko - who had accused Russia of making "unacceptable" demands - said rebels had launched an offensive after the agreement was announced.
The renewed fighting came after Ukrainian military officials said on Thursday that 50 Russian tanks, as well as armoured vehicles and rocket launchers, had crossed into Ukraine.
Ukraine and the West accuse Russia of supplying weapons and personnel to the rebels but Russia denies this.
More than 5,400 people have been killed since the conflict began. There has been a dramatic rise in casualties in recent days, with 263 civilians killed in populated areas between 31 January and 5 February.
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