Thousands of Syrian rebels have broken with the Western-backed coalition and called for a new Islamist front, undermining international efforts to build up a pro-Western military force to replace President Bashar al-Assad.

Ever more divided on a battlefield where Assad’s better-armed troops have been gaining ground, allies of the Free Syrian Army (FSA) were among 13 disparate rebel factions to disown the exile leadership and build an Islamic alliance that includes the al-Qaeda-linked Nusra Front, commanders have said.

Islamist front undermines efforts for a pro-Western military force

Details of the numbers of fighters involved and of how they would cooperate remained unclear.

But, in an online video, a leader of the Islamist Tawheed Brigade said the bloc rejected the authority of the Syrian National Coalition (SNC) and the Western- and Saudi-backed exile administration of Ahmad Tumeh.

A spokesman for Coalition president Ahmed Jarba, who was attending the UN general assembly in New York, said Jarba would head for Syria today to respond: “We are not going to negotiate with individual groups. We are going to come up with a better structure for organising the fighting forces,” the spokesman, Loay Safi, said. The move is a setback for foreign leaders trying to bolster more secular rebel groups and to reassure voters sceptical of deeper involvement in Syria’s civil war.

Some may think again about help for the fighters, which ranges from weaponry from the Gulf to non-lethal aid from Europe and the US. For Assad, already cheered by Russian diplomatic assistance that undermined US plans to bomb his forces following a poison gas attack, any more powerful rebel coalition could challenge his army’s resurgence in the field. But that could be more than offset by a weakening of international backing for his enemies.

Though some moderate Islamist fighters denied the move meant a more radical, sectarian approach, a more visible role for Islamist radicals at the expense of the SNC may bolster Assad’s argument that the alternative to his rule, based on his father’s military takeover four decades ago, is a Syria run by al-Qaeda.

The most hardline Islamist militant faction, al-Qaeda’s Islamic State of Iraq and the Levant (ISIL) which has brought growing numbers of foreign jihadists into Syria, was not a signatory to the new pact.

It was unclear, however, whether it had rejected involvement or had not been invited to join.

The 13 groups signed a statement calling for the opposition to Assad to be reorganised under an Islamic framework and to be run only by groups fighting inside Syria.

Signatories range from hardliners like the Nusra Front and Ahrar al-Sham to more moderate groups such as the Tawheed Brigade and Islam Brigade.

“These forces feel that all groups formed abroad without returning to the country do not represent them, so the forces will not recognise them,” said the statement by Abdulaziz Salameh, the political leader of the Tawheed Brigade.