The leader of one of Syria’s most prominent rebel units has died after being wounded during an attack by government troops last week, dealing another blow to fighters reeling from a series of recent battlefield losses.

The death of Abdul-Qadir Saleh, founder of the Tawhid Brigade followed advances by President Bashar Assad’s troops against rebels on two key fronts: the capture of a string of opposition-held suburbs south of Damascus and the taking of two towns and a military base outside the northern city of Aleppo.

The Tawhid Brigade is one of Syria’s best-known and powerful rebel groups, with an estimated 10,000 fighters, and is particularly strong in Aleppo province.

Under Saleh’s command, the group last year spearheaded a rebel push that seized large sections of the provincial capital Aleppo.

On Thursday night a government air strike hit its command post. The 34-year-old Saleh was severely wounded and later died in a hospital in Turkey.

The Tawhid Brigade was once part of the mainstream Free Syrian Army, considered to be the military wing of Syria’s exiled Western-backed opposition. But in September, the brigade broke away and later formed the Islamic Authority, a coalition of Islamic rebel groups, including one linked to al-Qaeda.

Saleh’s trajectory reflected that of many ordinary Syrians who joined the armed uprising against Assad’s rule. He was a married merchant who took part in peaceful demonstrations that began in March 2011.

After a violent crackdown by security forces, Syria’s conflict became an armed uprising and Saleh turned to guns.

He founded the Tawhid – or Monotheism – Brigade some 10 days before rebels overran and seized parts of Aleppo.

It was the latest setback for the rebels. Elsewhere in Syria, they are on the defensive in a high-stakes battle in Qalamoun, a mountainous area that stretches north of the Syrian capital along the Lebanese frontier.

It appears to be part of a long-anticipated government offensive aimed at cutting supply routes to rebel-held areas around Damascus, and cementing Assad’s hold on a key corridor from the capital to the coast.