BEIRUT — The leader of one of Syria’s most prominent rebel units died early Monday of shrapnel wounds sustained during shelling by government troops last week, his group said.

The death of Abdul-Qadir Saleh, founder of the Tawhid Brigade, was another blow to the rebels, reeling from a series of recent battlefield losses to President Bashar Assad’s forces.

Government troops have made headway against the rebels on two key fronts, capturing a string of opposition-held suburbs south of Damascus and taking two towns and a military base outside the northern city of Aleppo.

The Tawhid Brigade is one of Syria’s better-known and stronger rebel groups, with an estimated 10,000 fighters. It’s one of the main rebel groups in Aleppo province. Under Saleh’s command, the group last year pushed into the provincial capital, Aleppo, seizing large sections of the city for the rebels.

On Thursday night, government forces targeted its command post in Aleppo province. The 34-year-old Saleh was severely wounded and later died in a hospital in Turkey, said a brigade spokesmen who goes by the name of Akram al-Halaby. Many rebels do not use their real names, fearing they or their families would be identified and targeted by security forces

The shelling also killed the brigade officer, Abu Tayeb, and wounded another spokesman, Saleh Anadan.

Saleh’s body was quickly returned to Syria and he was buried in his hometown of Marea in Aleppo province, al-Halaby said. The brigade’s political chief, Abdul-Aziz Salameh, who was lightly wounded in Thursday’s shelling, was appointed to succeed Saleh, the spokesman said.

The Tawhid Brigade was once a part of the 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-Qaida.

Saleh’s death was also confirmed Monday by Rami Abdurrahman of the British-based Syrian Observatory for Human Rights, which collects information from a network of activists on the ground.

Saleh’s trajectory reflected that of many rebels 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 Unity — Brigade some 10 days before rebels seized parts of Aleppo, al-Halaby said.

A video uploaded to social media networks in 2012 shows Saleh flinching as a bomb drops near his headquarters in Aleppo province. His comrades call on him to rush inside a building for protection, but he insists on standing outside, saying: “Nobody dies until God gives him his life, and his date of death.”

Syria’s rebels are currently on the defensive in a high-stakes battle in the mountainous region of Qalamoun on the Lebanese border. There, government forces are trying to cut off a supply route for rebels, centered around the town of Qara.

But the rebels are still capable of carrying out large attacks and the opposition remains firmly entrenched in other areas around Damascus. A massive explosion Sunday leveled a government office in the northeastern suburb of Harasta, killing at least 31 soldiers, according to the Observatory.

Three brigadiers and one major general were among the dead, according to the Observatory’s Abdurrahman. There was no confirmation from government officials or state media on the attack.

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