A room in a high school in Raqqa after it was reportedly hit during an airstrike Sunday by forces loyal to President Bashar al-Assad, according to activists. Nour Fourat/Reuters

At least 16 people, most of them students, have been killed in an air attack that hit a high school in the rebel-held Syrian city of Raqqa, opposition activists say.

Graphic amateur video said to be filmed on Sunday in Raqqa showed several bloodied bodies strewn across a dirt yard, some torn in half.

Opposition activists based in Raqqa, a northeastern city of about 250,000 people, said there were more than 30 people wounded in addition to the fatalities.

Al Jazeera could not independently verify the reports because of reporting restrictions in Syria.

Raqqa has been under the control of fighters battling to overthrow President Bashar al-Assad since March, but the city remains subject to regular aerial bombardment by government forces.

Farther south, government forces shelled near the Ramtha border crossing in Deraa near Jordan on Sunday, a day after rebels — including fighters affiliated with al-Qaeda — seized control of the post, which had served as the customs office on the border with Jordan.

Al Jazeera's Nasser Shadid, reporting from Deraa, said that while the border crossing was now under the control of the rebels, the surrounding area was still controlled by regime troops.

"Clashes are continuing between the two sides," he said.