The city of Donetsk in eastern Ukraine was shelled overnight, despite a cease-fire signed by the government and pro-Russian separatists late last week.
Witnesses say Ukrainian positions in the rebel-held city were hit by mortar and rocket fire four times during the night.
It was unclear if there were casualties.
The cease-fire has largely held across eastern Ukraine since taking effect on Friday. But fighting has been reported in parts of the region, especially in the industrial hub of Donetsk and near the port city of Mariupol.
On Tuesday, the Interfax news agency reported that four Ukrainian servicemen have died since the cease-fire was signed. Defense ministry official Vitaly Andronaty also told Interfax that 29 people have suffered wounds.
Both sides blame the other for violating the truce.