Jihadi militants in Syria including al-Qaeda are mobilizing again for all-out war against President Bashar al-Assad, taking advantage of the collapse of peace talks to eclipse nationalist rival insurgents that signed on to a faltering truce, Reuters says.

Al-Qaeda's Syrian branch, the Nusra Front, was excluded from a ceasefire put in place in February and from peace talks that followed. The talks broke up last month, with Assad's government and foes blaming each other for military escalation.

After lying low in the early days of the truce, Nusra has re-emerged on the battlefield as the diplomacy has unraveled, spearheading recent attacks on pro-government Iranian militias near Aleppo, Nusra commanders and other rebels say. In the latest expansion of its profile, it and other groups have revived the Jaish al-Fatah, or the army of conquest, a military alliance of disparate Islamist rebel groups that won big victories against government forces last year, Reuters says.

Nusra's resurgence could undermine the Western-backed rebel groups that signed up to the truce and attended the peace talks, and gives Assad's government and its Russian and Iranian backers more reasons to press on with a war during which they have hit insurgents of all stripes.

"Jaish al-Fatah has returned, but this time in strength, and our goal is to spread to the major fronts in Syria," said Abu Shaimaa, a Nusra Front commander, speaking to Reuters from rebel-held Idlib province, of the revival of the Islamist rebel alliance.

"We ask God that with Jaish al-Fatah's return, the victories will also return," added Zaher Abu Hassan, head of a Jaish al-Fatah media organization in Idlib.

The Islamist rebels still face the challenge of overcoming their own rivalries. One senior insurgent source said that while Jaish al-Fatah had made a comeback in one area, talks were still underway to relaunch the alliance more widely.

"In southern Aleppo, yes there is an operations room, but the goal is (to repeat it) on all the active fronts," he said.