The Ukrainian military says five of its soldiers have died in fighting just north of Donetsk – the second-highest death toll in one day this year.
Pro-Russian separatist rebels killed four in an ambush at Verkhnetoretskoye, near Avdiivka. The fifth died when his vehicle hit a landmine.
Ukrainian military spokesman Oleksandr Motuzyanyk confirmed the death toll. Early on Monday rebel mortars targeted Ukrainian troops in the same area.
Reports say two rebels were killed.
A spokesman for the self-styled “Donetsk People’s Republic (DPR)”, Eduard Basurin, said the two died during Ukrainian shelling of DPR positions.
Speaking on Sunday, he said the rebels had blocked a Ukrainian army attempt to break through between Avdiivka and Yasynuvata.
The Ukrainian army losses were the highest for a single day since the deaths of seven in fighting last Tuesday.
The continuing use of heavy weapons along the front line in eastern Ukraine is undermining a fragile ceasefire.
International monitors from the OSCE security organisation have been prevented from reaching many areas in the conflict zone.
Both sides are still far off fulfilling the Minsk peace deal they signed up to.
Nato, Western leaders and the Ukrainian government accuse Russia of sending heavy armour and regular troops to the rebels. Moscow denies that, but admits that Russian “volunteers” are helping the rebels.