Abdel Qader al-Saleh
© Associated PressIn this Aug. 13 citizen journalism image provided by the Tawheed Brigade and distributed through the Associated Press, Abdel Qader al-Saleh, left, speaks to his fighters ahead of an attack on government troops, in Aleppo, Syria.
The commander of the largest rebel brigade in northern Syria was killed Thursday, his group said Monday, in a setback for rebels increasingly struggling to maintain territory and make gains against the Syrian army and its allies.

Abdel Qader al-Saleh was the leader and one of the founders of the Islamist Tawheed Brigade, which numbers an estimated 4,000 fighters and is considered among the most effective on the ground, according to analysts, rebels and activists.

Mr. Saleh was hit in a government air attack on a Tawheed Brigade barracks north of Aleppo. He died in a hospital in Turkey, other commanders said.

The Syrian army has gained significant territory in recent weeks in areas near the capital Damascus and are making gains in the country's largest city of Aleppo, including breaking a monthslong rebel siege of Aleppo's airport.

Mr. Saleh was 33 years old and hailed from the town of Marea, 30 kilometers north of Aleppo. Rebels in the area said that he served as a Muslim missionary in various countries after completing his Syrian military service. Before the Syrian uprising, he worked in agricultural trade.

Mr. Saleh was an Islamist and hoped for the establishment of an Islamic state in Syria, but once said in Al Jazeera interview: "This state will not be imposed by force."

Rebels and activists said his death was a severe blow, especially to their morale.

"I swear to God, I feel like we have been broken," said one activist who spent a lot of time on the battlefield, as a citizen reporter, with Mr. Saleh.

Colleagues of Mr. Saleh hailed him as humble, kind and shrewd. Abdeljabar Ughaidi, a defected army colonel and a commander for rebels in Aleppo, said by phone: "We undoubtedly lost a hero of the Syrian revolution. He was a noble warrior who was a force to be reckoned with on the ground."

In one video posted online, Mr. Saleh is seen speaking in an interview when a building within clear view is hit by government planes. While the rest scramble to go and urge him to leave, Mr. Saleh looks at the camera and says: "These are Assad's bombers. We will remain steadfast even if we only have this light Russian weapon."

(Mohammad Nour Alakraa in Beirut contributed to this article.)