Ukrainian officials and Russian-backed separatists accused each other of violating a holiday cease-fire on December 23, just hours after the truce came into effect in Ukraine's conflict-torn east.
Army spokesman Andriy Lysenko said "illegal armed groups have already violated these agreements."
"Since the beginning of the day we recorded seven enemy shellings of our positions," Lysenko told journalists.
Separatist authorities rejected the claims and accused Ukrainian forces of "continuing to shell" their territory with mortars and also of using small arms against them.
The two sides had agreed on December 22 to halt all fire and maneuvers starting from midnight to last through the Christmas and New Year's holidays.
Martin Sajdik, the head envoy from the Organization for Security and Cooperation in Europe (OSCE), said the two sides had also agreed to limit their troop and tank movements along the frontline.
Most Orthodox Christians in Ukraine and Russia celebrate Christmas on January 7.
The United Nations says more than 9,000 people -- most of them civilians -- have died since the conflict in eastern Ukraine began in April 2014.