Ukraine crisis: Poroshenko says peace deal in danger
Petro Poroshenko also accused Russia of "significantly increasing" its offensive despite the peace agreement reached in Minsk on Thursday.
Meanwhile, the US said it was very concerned by reports of heavy weapons coming across the border from Russia.
Shelling was heard in the rebel-held city of Donetsk early on Saturday.
Fierce battles are also said to be continuing around Debaltseve, a strategic government-held town almost encircled by rebel forces.
The BBC's David Stern in Kiev says that as the clock ticks down to the ceasefire, fears are rising that it won't be observed.
More than a dozen civilians are said to have died in shelling in eastern Ukraine on Friday.
It is unclear who was behind the shelling but both the Ukrainian military and pro-Russian rebels in the eastern Donetsk and Luhansk regions accuse each other of targeting residential areas.
The presidents of France, Russia and Ukraine as well as the German chancellor - who together clinched the agreement in the Belarusian capital - are due to discuss the issue by phone over the weekend.
The UN Security Council will also meet in emergency session on Sunday.
'Escalation expected'
Correspondents say the fighting shows no sign of stopping.
Ukrainian Deputy Defence Minister Petro Mekhed said the rebels wanted to "raise their flag" over Debaltseve and the key port city of Mariupol before the midnight ceasefire (22:00 GMT) kicked in.
"Ukraine is expecting an escalation and taking all necessary measures to be able to respond," Mr Mekhed told reporters.Rebel shelling killed two people in a cafe in Shchastya, near Luhansk, on Friday as well as a child near a school in Artemivsk, a town near Debaltseve, according to Kiev-controlled regional authorities.
Meanwhile, the rebels said at least six people had died in shelling in the city of Donetsk and town of Horlivka. The rebels accuse government forces of shelling the towns.
"After what we achieved in Minsk this is not just shelling of Ukrainian civilians and residential neighbourhoods - this is an attack on our Minsk achievements, without any explanations," President Poroshenko said.
"Unfortunately, after Minsk, Russia's offensive operations have intensified," he said, before adding: "We are still convinced that the Minsk achievements are in a big danger."
There were also reports of a government offensive near Mariupol, the city between rebel-held eastern areas and the southern Crimea peninsula, which was annexed by Russia last March.
'Serious live fire'
The group responsible for monitoring the ceasefire said it remained hopeful, despite there being "quite serious live fire" in several areas on Friday.
"We feel that the Minsk agreements are really the only available roadmap to a sustainable ceasefire," Michael Bociurkiw, spokesman for the OSCE, told the BBC.
In another development, a consignment of armoured vehicles from the UK has been delivered to Ukraine, the Ministry of Defence confirmed.
It said they were out-of-service, unarmed vehicles and were not lethal equipment.
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The latest Minsk agreement:
Ceasefire to begin at 00:01 local time on 15 February (22:01 GMT 14 February)
Heavy weapons to be withdrawn, beginning on 16 February and completed in two weeks - beyond a buffer zone behind the current front line for Ukrainian forces and behind the September front line for separatist forces
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
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Pro-Russian rebels signed the peace agreement but key issues remain to be settled, including the situation in Debaltseve.
European leaders have warned Russia that it could face additional sanctions if the agreement is not respected.
German Chancellor Angela Merkel said the deal offered "a glimmer of hope", but warned: "It is very important that words are followed by actions."Ukraine and the West accuse Russia of supplying weapons and personnel to the rebels but Russia denies this.
Officials say more than 5,400 people have been killed since the conflict began, but the UN believes the actual death toll to be much higher.
There has been a dramatic rise in casualties in recent days, with 263 civilians killed in populated areas between 31 January and 5 February.