Kiev military spokesman Leonid Matyukhin told AFP that another six soldiers were wounded in the battle for control of the strategic city of Debaltseve.
The railroad hub on the edge of the war zone in January 2015 was the scene of one of the deadliest confrontation in the 31-month war.
"Today, the rebels staged a massive attack on our positions in the Debaltseve region," Matyukhin told AFP by telephone.
He said the firefight lasted for two hours and involved artillery and large-calibre grenade launchers.
"Unfortunately, five of our fighters were killed and six injured," the Ukrainian military spokesman said.
The January 2015 Debaltseve battle was won by the separatists and led to peace talks the following month that drew in the leaders of Germany and France.
The February 2015 deal that was also signed by Ukrainian President Petro Poroshenko and Russia's Vladimir Putin was meant to end one of Europe?s bloodiest conflicts in decades by the end of that year.
But the so-called Minsk Agreements have been repeatedly broken by both sides and low-scale warfare continues in the European Union's backyard in a conflict that has claimed some 10,000 lives.
A sudden military flare-up in Ukraine would add headaches to EU leaders who are already grappling with Britain's decision to leave the bloc and Donald Trump's unexpected election as president of the United States.
Matyukhin said that 20 insurgents were killed and 30 wounded in Sunday's battle.
But both sides often exaggerate the others' losses and rebel spokesman Eduard Basurin said the number of his fighters killed remained unknown.
Basurin accused Kiev's forces of trying to win back positions they lost nearly two years ago.
"We beat them back," Basurin told AFP. "I have no information about how many casualties we suffered."
Ukraine and its Western allies accuse Russia of taking an active part in the war in retaliation for Kiev's February 2014 ouster of a Moscow-backed president and tilt toward the West.
Russia flatly denies the charges and calls any of its soldiers killed or captured in the war zone volunteers.
But both the United States and the European Union have imposed economic sanctions for Russia's actions in Ukraine and the March 2014 annexation of the Crimea peninsula.
The Kremlin responded by banning the import of most Western food.
That step has proven to be unpopular with some European farmers in nations such as Italy and Spain.
But the EU on Thursday extended its economic punishment on Russia by another six months.