Two teenagers died and four were wounded when an artillery shell hit a school playing field as they played football in eastern Ukraine.
The attack in Donetsk came as both sides in the Ukrainian conflict accused each other of tearing up a peace deal.
A fragile ceasefire has been in place since 5 September, although hundreds of people have been killed since then.
Fighting erupted in April after pro-Russian separatists seized control in the regions of Donetsk and Luhansk.
More than 4,000 people have died in eastern Ukraine since the conflict began and the European OSCE monitoring mission has warned that the "bloodletting" is still going on, with numerous incidents of shelling.