A Syrian air strike has killed a senior commander of the Islamist Liwa Al-Tawhid rebel brigade in Aleppo and wounded its chief and another leader, a watchdog said Friday.

Four other rebel chiefs were killed in separate incidents, two in the northern Aleppo province and the others in the south of the country, the Syrian Observatory for Human Rights said.

Yussef Al-Abbas, known as Abu Al-Tayyeb, was intelligence chief for Liwa Al-Tawhid and was killed in a strike Thursday on an army base captured by the rebels a year ago, the Britain-based Observatory said.

He had been in a car along with the brigade's top commander, Abdelkader Saleh, and another senior figure of the group, Abdelaziz Salameh, both of whom were wounded.

Following the attack, Liwa Al-Tawhid arrested 30 people suspected of being informers for the regime of President Bashar Al-Assad.

The powerful brigade is among a number of Islamist units that have rejected the mainstream opposition Syrian National Coalition.

In separate incidents, two chiefs of rebel battalions were killed in fighting with loyalist forces near the international airport outside Aleppo.

Elsewhere in the province, a former army colonel who commanded another rebel brigade was killed in fighting in the Maarat al-Artiq area.

Jihadist fighters in Aleppo have called for mass mobilisation to counter regime advances after the Islamists suffered a string of setbacks around the northern city.

And in Homs province, to the south of Aleppo, a rebel chief was killed in Mahine, where a major army operation is under way to recapture a large cache of arms seized by the insurgents.

An estimated 120,000 people have been killed and millions displaced by Syria's civil war, which erupted after a fierce government crackdown on pro-democracy protests first held in March 2011.