Four soldiers have been killed and five wounded after Ukraine's military regained control of a checkpoint in the eastern region that had been earlier taken over by separatists.
The fighting in Donetsk erupted overnight on Friday in violation of a ceasefire agreement between the government and pro-Russian rebels that was to last until Monday.
Earlier on Friday, Ukraine's president, Petro Poroshenko, decided to extend the ceasefire, on the same day that he signed a trade agreement with the EU - the deal that sparked the political crisis last year.
The ceasefire extension had been undertaken, it said, in line with a deadline set by EU leaders for Ukrainian rebels to agree to ceasefire verification arrangements, return border checkpoints to Kiev authorities and free hostages, including monitors of the OSCE rights and security watchdog.
At a separate meeting, Poroshenko and national security chiefs said that during the next 72 hours recruitment centres
for Russian fighters across the border in Russia should be closed.
The one-week truce had been due to expire on Friday at 7pm GMT, and will now expire at 7pm GMT on June 30, according to the presidential website.
Some 110,000 people have fled to Russia from Ukraine while more than 54,000 have been displaced inside the conflict-torn country, the UN said on Friday.
"Since the start of 2014, 110,000 Ukrainians have arrived in Russia," said Melissa Fleming, a spokeswoman for the UN's refugee agency.
She said that most had fled from the embattled eastern regions of Donetsk and Luhansk, where Ukrainian forces are battling separatists, AFP news agency reported.
But she underlined that it was not possible to say whether most or all of those fleeing to neighbouring Russia were from Ukraine's Russian-speaking population.
Most people are seeking other forms of legal stay, often because they're concerned about complications involving seeking asylum or since there might be reprisals if they return to Ukraine
Claims that Russian-speakers in Ukraine are under threat have been cited regularly by the rebels and Moscow, though UN human rights probes have said there is little evidence for such fears.
"Only 9,500 have requested asylum. Most people are seeking other forms of legal stay, often because they're concerned about complications involving seeking asylum or since there might be reprisals if they return to Ukraine," Fleming said.
Most of the most recent arrivals are clustered in the western Russian cities of Rostov-on-Don and Bryansk, near the Ukrainian border, said Fleming.
Close to 13,000 people, including 5,000 children, are being accommodated mainly in public buildings and tented camps in Rostov-on-Don.
In Bryansk, where 6,500 arrived in recent days, the majority are staying with friends and relatives.
Within Ukraine itself, 16,400 people have fled their homes in the east in the past week alone, taking the total number of internally-displaced to 54,400.
"The rise in numbers of the past week coincides with a recent deterioration of the situation in eastern Ukraine. Displaced people cite worsening law and order, fear of abductions, human rights violations and the disruption of state services," Fleming said.