Syrian warplanes on Wednesday bombarded a rebel-held district of the city of Aleppo, causing dozens of casualties, as soldiers repelled an attack on a nearby army base, a monitoring group said.

"Dozens of people were killed or injured by an air strike on a building in the district of Shaar" in eastern Aleppo, said the Syrian Observatory for Human Rights, without giving a precise toll.

"The field hospital was badly damaged by the strike," the Britain-based monitoring group added.

Amateur video filmed in Shaar and distributed by the Observatory showed people gathering around the destroyed building, which was reduced to a mountain of grey rubble.

Also in the northern province of Aleppo, the army repelled a rebel attack on a military base at Sheikh Suleiman, 25 kilometres (15 miles) northwest of the city, killing at least 25 insurgents, the Observatory said, citing rebels.

The area was mined and came under aerial bombardment, it said.

Rebel fighters have besieged the base for several weeks but have now been forced to pull back as forces loyal to President Bashar al-Assad recover ground in northwest Syria.

In contrast, rebels on Wednesday dragged out the last corpses of soldiers from ruined buildings and gathered the booty of arms and ammunition in the sprawling army garrison of Base 46, also in Aleppo province.

Rebel fighters seized the hilltop base on Sunday.

Defected General Mohammed Ahmed al-Faj, who commanded the assault, hailed the capture as "one of our biggest victories since the start of the revolution" against Assad's regime.

"Nearly 300 regime troops were killed in the fighting," the rebel commander told AFP at the scene. "Some 60 others were taken prisoner, and they will be tried soon."

Base 46 was one of the army's last remaining bases in the area bordering Turkey, which supports the revolt against Assad.

Rebels had aimed to also expel the army from Sheikh Suleiman, as they edge towards declaring the "liberation" from regime hands of northern and northwest Syria.

In violence in the capital on Tuesday, a mortar shell smashed into an upmarket district that houses several embassies, killing one person and injuring several others, the Observatory and Syria's official media said.

The blast in Abu Rummaneh marked the first time the wealthy district has been targeted since the March 2011 outbreak of the revolt.

On Wednesday, the army shelled the southern belt of Damascus and the town of Daraya southwest of the capital, said the Observatory, which relies on a network of activists, doctors and lawyers to compile and verify its data.

According to a preliminary toll compiled by the Observatory, at least 35 people were killed in violence across Syria on Wednesday, among them 16 civilians, five regime troops and 14 rebel fighters.

At least 39,000 people have been killed across Syria since the outbreak of the revolt, according to the watchdog.