Mortar rounds slammed into a school Wednesday in a rebel-held suburb east of Damascus, killing at least 13 children whose limp, bloodied bodies were later laid out on the floor of a crowded field hospital awaiting burial, activists said.

Syrian children often have been the victims of the country's civil war, now in its fourth year, but they rarely are specifically targeted. Still, Wednesday's attack in the town of Qaboun marked the most serious violence against Syrian minors since a twin suicide bombing killed at least 25 children in a government-controlled neighborhood in the central city of Homs in October.

Three mortar shells struck the Haya School in Qaboun before noon, said a local activist who uses the name Abu Akram al-Shami. Another activist based near Damascus, Amar al-Hassan, also reported the incident, as did Rami Abdurrahman of the Britain-based Syrian Observatory For Human Rights.

Abdurrahman said 13 children were killed, but the number likely would rise. A local activist collective, the Qaboun Media Office, put the death toll at 17 children. Conflicting casualty figures are common after such incidents.

In an amateur video posted online, the lifeless bodies of at least nine of the children with bloodied clothes lie on a white tile floor. Streaks of blood stain the floor. In another video, a woman breaks through the crowd at the medical facility, beating her chest and wailing in grief and shock. "My son, my son!" she weeps.

The videos appeared genuine and corresponded to Associated Press reporting of the incident.

It was not immediately clear who fired the mortar rounds. Both forces loyal to President Bashar Assad and rebels opposed to his rule use the weapons. Activists said they believed pro-Assad forces were responsible -- if only because it was unlikely that rebels would fire at their own people. Government officials had no immediate comment.

Rebels and government forces struck a truce in Qaboun some five months ago, and the town had not -- until Wednesday -- experienced any serious violence since. The town hosts thousands of Syrians forcibly displaced from other rebel-held areas.

Also Wednesday, in the Kurdish-dominated far north and northeastern regions, Kurdish forces distributed leaflets to residents, ordering them to report for compulsory military service, the Observatory said. Syrian Kurds run their self-declared, largely autonomous area, called Rojava.

They have been on the front line of the fight against militants of the Islamic State group.

Their battle against the extremists in the Syrian border town of Kobani has captured international attention, and the U.S. has assisted the fighters with airstrikes. From Monday through Wednesday, the U.S.-led coalition conducted four airstrikes in Syria, including three near Kobani, U.S. Central Command said.

Senior Syrian Kurdish official, Anwar Muslim said the move was necessary to repel the extremists.

"We want all of the people to come, train and learn to carry weapons, without discriminating between men and women. We need all people to learn how to carry weapons in order to be able to defend themselves and protect their areas and their villages," Muslim told the AP from Irbil, the capital of the largely autonomous Kurdish area of northern Iraq.