At least 10 people were killed on Wednesday when shells hit a school playground and a minibus on a nearby street in Donetsk, in eastern Ukraine, city authorities and witnesses said.
A shell hit a playground near school No. 57 on the first day of the school year in Donetsk. No children were among those killed.
Witnesses and city officials told Reuters the four dead included two teachers and a parent.
Terrified children were forced to shelter in the basement as the shelling yielded the highest daily civilian toll since a tenuous cease-fire was struck between Kyiv and pro-Russian separatists last month.
The pro-Kyiv regional government of Donetsk, which is now based in the government-controlled city of Mariupol, accused pro-Russian separatists of the Donetsk People's Republic of shelling the school.
"The Donetsk People's Republic used rocket launchers to shoot at a school ... the shell exploded five meters away from the building," the regional administration said in a statement.
The rebels, however, said the shelling came from positions held by government troops
First day of school
A source in Donetsk city hall told the French news agency AFP that the strike happened right after the school's 70 pupils lined up for an assembly to mark the first day of class - held nationally on September 1 but pushed back by rebel authorities because of the continued violence in the area.
Reuters correspondents also saw six bodies in a burnt-out minibus and on streets nearby.
Organization for Security and Cooperation in Europe (OSCE) observers and armed pro-Russian rebels were inspecting the scene.
Donetsk, with a population of about 1 million, is the largest rebel-held city in Ukraine. Rebels hold key points inside the city itself.
The United States earlier decried the "intensifying violence" in eastern Ukraine and called on Russia and the rebels to hold fire.
"Since the cease-fire was signed ... attacks on Ukrainian positions and towns, including around the Donetsk airport ... have killed and wounded scores of Ukrainian armed forces and civilians," State Department spokeswoman Jen Psaki said.
"We call on Russia and the separatists it backs to immediately end these attacks," Psaki said.
Shelling at airport
Meanwhile, rebels also appear to be successfully closing in on the government-held airport of Donetsk, a strategic victory for the pro-Russian separatists that further undermines a shaky cease-fire in the region.
Russia's Itar-Tass news agency quoted the the prime minister of the separatists' self-declared Donetsk People's Republic, Alexander Zakharchenko, as saying that the rebels now control 90 percent of the Donetsk airport, which was previously in government hands and has been the focus of the worst fighting in the region for weeks.
From a distance, it was clear that artillery fire hitting the airport was coming from government-held positions outside the city - an indication that it may no longer be under Kyiv's control, the AP reported.
Violence has continued in eastern Ukraine despite a cease-fire between Kyiv and the separatists last month. The sides also agreed to set up a 30 kilometer buffer zone away from the front lines during recent peace talks in Minsk, Belarus.
At least 3,500 people have been killed in eastern Ukraine since fighting began in April after Russia annexed Ukraine's Crimean peninsula.
The government is hoping the tenuous cease-fire can hold through October 26, when the country holds parliamentary elections.
Some material for this report came from Reuters, AFP and AP.