Islamic State (IS) is moving towards a new alliance with Syria's largest al-Qaeda-allied group that could help offset the threat posed by US-led air strikes.
The al-Nusra Front vowed retaliation for the strikes, the first wave of which a week ago killed scores of its members. Many al-Nusra units in northern Syria appeared to have reconciled with IS, with which it had fought bitterly early this year.
A senior source confirmed on Sunday that al-Nusra and IS leaders were holding war-planning meetings. While not yet formalised, the addition of at least some al-Nusra numbers to IS would strengthen the group's ranks and further its reach at a time when air strikes are crippling its funding sources and slowing its advances in both Syria and Iraq.
The al-Nusra Front, which has direct ties to al-Qaeda's leader, Ayman al-Zawahiri, denounced the attacks as a "war on Islam". A senior al-Nusra figure said that 73 members had defected to IS last Friday alone.
"We are in a long war," said Abu Firas al-Suri, a group spokesman, on social media. "This war will not end in months nor years, this war could last for decades."
In the rebel-held north there is resentment among Islamist units of the Syrian opposition that the strikes have done nothing to weaken the Syrian regime.