According to Sky News, President of Ukraine has warned that fresh shelling in the east of the country is putting the impending ceasefire in danger.
At least 13 civilians have been killed and dozens wounded in fighting in the last 24 hours in the east of the country.
Seven Ukrainian service personnel have also been killed as the new peace deal brokered by Germany and France in Minsk is due to come into force at 10pm UK time.
Sky’s Senior Correspondent Stuart Ramsay, in Kiev, said an explosion had been reported in central Donetsk on Saturday, with two people killed.
He said the fighting could be a “sort of last-ditch land grab by the two sides to try and get their positions that they have been trying to get to for the last week or so, try and make it before the deadline, and then observe the ceasefire.”
The fiercest fighting is said to be taking place around the strategic railway town of Debaltseve, which has come under sustained artillery fire.
Petro Poroshenko says the Minsk agreement is in “big danger”, as 13 civilians and seven Ukrainian soldiers are killed in 24 hours.
Fierce clashes have also been reported near the port city of Mariupol.
Ukrainian President Petro Poroshenko said: “After what we reached in Minsk, this is not just an attack of Ukrainian civilians, on Ukrainian living centre.
“This is an attack on Minsk results and without any explanations because, unfortunately after Minsk the offensive operation of Russia intensified insignificantly.
“And we still think that the Minsk agreement is under big danger.”
The ceasefire envisages a withdrawal of the heavy weapons responsible for many of the 5,000 casualties in the conflict that broke out almost a year ago.
Ukraine and the West have accused Russia of pushing weapons and troops into the country to bolster the separatist push to seize more territory in the east of the country.
Moscow has repeatedly denied the charges, but the rebels have the modern heavy weaponry of a regular army.
Mr Poroshenko is due to speak with US President Barack Obama, German Chancellor Angela Merkel and French President Francois Hollande later on Saturday. The UN Security Council is also expected to meet on Sunday for an emergency session.
Mr Obama has warned he could start weapons supplies to Ukraine if the new peace deal collapses. Ms Merkel has also threatened further sanctions on Russia if the truce fails.
Both sides have to begin withdrawing heavy weapons from the frontline within two days of the ceasefire, which will establish a buffer zone of between 31 and 87 miles wide.
The Organisation for Security and Co-operation in Europe will monitor the ceasefire and will increase the number of observers it has in the country to 350.
