Ukraine's president has
warned that a deal to end fighting in the east is in "great danger"
after heavy fighting ahead of Saturday night's ceasefire.
Petro Poroshenko also accused Russia of "significantly increasing" its offensive despite the peace agreement reached in Minsk on Thursday.
The US ambassador to Ukraine tweeted a satellite image that he said showed Russian artillery near Debaltseve.
Shelling was heard in the rebel-held city of Donetsk early on Saturday.
Fierce battles are said to be continuing around Debaltseve - a strategic government-held town almost encircled by pro-Russian rebel forces - and outside the southern port city of Mariupol.
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.
Ukraine's military said on Saturday morning that seven service personnel had been killed and 23 wounded over the past 24 hours.
"Ahead of midnight (time of planned ceasefire) rebels are trying to complete tactically important plans to enlarge the territory under their control," spokesman Andriy Lysenko.
More than a dozen civilians were said to have died in shelling in Ukraine's eastern Donetsk and Luhansk regions on Friday.
The Ukrainian military and pro-Russian rebels accuse each other of targeting residential areas.
Rebel fighters have continued to besiege the government-held town of Debaltseve
The presidents of France, Russia and Ukraine as well as the
German chancellor - who together clinched the agreement in Minsk, 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.
Meanwhile, the US has said it is very concerned by reports of heavy weapons coming across the border from Russia.
The US ambassador to Ukraine, Geoffrey Pyatt, also said on Twitter that Russian units along the border were preparing a large shipment of supplies to separatist fighters.
He pointed to satellite images of what he said was Russian artillery north of Debaltseve, taken on Thursday.
'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 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.
Ukraine's volunteer Azov battalion said pro-Russian rebels were using artillery and tanks to attack Shyrokyne, a village near Mariupol, on Saturday morning.
The port city lies between rebel-held eastern areas and the southern Crimea peninsula, which was annexed by Russia last March.
A young child was killed in Artemivsk in what the local authorities said was shelling by rebels
Government forces were also reported to have retaken a number of villages around Mariupol
President Poroshenko said despite the agreement there was a "long way to go to peace"
Speaking earlier, President Poroshenko said that despite what
had been agreed in Minsk, "Russia's offensive operations have
intensified".
"We are still convinced that the Minsk achievements are in a big danger," he added.
'Serious live fire' The group responsible for monitoring the ceasefire said it remained hopeful, despite there being "quite serious live fire" in several areas.
"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.
Petro Poroshenko also accused Russia of "significantly increasing" its offensive despite the peace agreement reached in Minsk on Thursday.
The US ambassador to Ukraine tweeted a satellite image that he said showed Russian artillery near Debaltseve.
Shelling was heard in the rebel-held city of Donetsk early on Saturday.
Fierce battles are said to be continuing around Debaltseve - a strategic government-held town almost encircled by pro-Russian rebel forces - and outside the southern port city of Mariupol.
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.
Ukraine's military said on Saturday morning that seven service personnel had been killed and 23 wounded over the past 24 hours.
"Ahead of midnight (time of planned ceasefire) rebels are trying to complete tactically important plans to enlarge the territory under their control," spokesman Andriy Lysenko.
More than a dozen civilians were said to have died in shelling in Ukraine's eastern Donetsk and Luhansk regions on Friday.
The Ukrainian military and pro-Russian rebels accuse each other of targeting residential areas.
Rebel fighters have continued to besiege the government-held town of Debaltseve
The UN Security Council will also meet in emergency session on Sunday.
Meanwhile, the US has said it is very concerned by reports of heavy weapons coming across the border from Russia.
The US ambassador to Ukraine, Geoffrey Pyatt, also said on Twitter that Russian units along the border were preparing a large shipment of supplies to separatist fighters.
He pointed to satellite images of what he said was Russian artillery north of Debaltseve, taken on Thursday.
'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 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.
Ukraine's volunteer Azov battalion said pro-Russian rebels were using artillery and tanks to attack Shyrokyne, a village near Mariupol, on Saturday morning.
The port city lies between rebel-held eastern areas and the southern Crimea peninsula, which was annexed by Russia last March.
A young child was killed in Artemivsk in what the local authorities said was shelling by rebels
Government forces were also reported to have retaken a number of villages around Mariupol
President Poroshenko said despite the agreement there was a "long way to go to peace"
"We are still convinced that the Minsk achievements are in a big danger," he added.
'Serious live fire' The group responsible for monitoring the ceasefire said it remained hopeful, despite there being "quite serious live fire" in several areas.
"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.
