Ukraine says its army has repulsed an offensive by pro-Russian rebels in the east who sought to seize territory near the strategic town of Debaltseve.
The Ukrainian military command said five of its troops and 25 rebels were killed in heavy fighting on Sunday.
The figures could not be independently verified. It is the most serious flare-up for several weeks on the front line.
The rebels said they had thwarted a Ukrainian army assault near Debaltseve, killing 10 troops and wounding 20.
The rebels also destroyed two infantry fighting vehicles, according to a commander in rebel-held Donetsk, Eduard Basurin.
The spokesman for rebels in neighbouring Luhansk region, Maj Andriy Marochko, said "fighting is under way along the entire line of contact in the Debaltseve sector".
He said the Luhansk "people's militia" was being shelled by artillery and mortar units but was preventing the Ukrainian army from reaching villages.
Earlier, the Ukrainian army said the rebels had tried - and failed - to outflank the government troops at Svitlodarsk, near Debaltseve.
Ceasefire monitors from the OSCE security organisation confirmed "heavy fighting" in the Debaltseve-Svitlodarsk area.
On Sunday, their report said, OSCE monitors heard 680 artillery rounds and 20 tank rounds in a six-hour period. There was also a temporary power cut in Svitlodarsk.
Ukrainian military spokesman Andriy Lysenko, quoted by the news website 112.ua, said the loss of five soldiers was the heaviest toll in five months of sporadic clashes on the front line.
The terms of the Minsk ceasefire deal, agreed between the government and rebel sides in February 2015, are being violated daily, the OSCE reports.
OSCE monitors complain that both sides are preventing them from reaching parts of the conflict zone, hampering the monitoring mission.
Ukrainian government troops pulled out of Debaltseve in February 2015 after heavy fighting.
Eastern Ukraine's unrecognised separatist republics - based in Donetsk and Luhansk - are international pariahs and economically dependent on Russia.
Kiev, Nato and Western leaders accuse Russia of supplying the rebels with heavy armour and regular troops. Moscow denies that, while admitting that some Russian "volunteers" are fighting for the rebels.
The Ukrainian military headquarters commander, Gen Alexander Sirsky, said his troops had come under heavy rebel artillery fire. He said there had been three bombardments in the Svitlodarsk area, lasting up to six hours each time.
The shelling came from Kalinovka, Vuglehirsk and Debaltseve, he said, accusing the rebels of using civilian areas as cover, in violation of the Minsk accord.
The Ukrainian military said there had also been renewed rebel shelling of Shyrokyne, a strategic village near the government-held port city of Mariupol.