Thousands of Syrian rebels have broken with the Western-backed coalition to form a new Islamist force that may pressure Syrian President Bashar al-Assad but also undermine the West's hopes of putting its own allies in his place.

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

Details of the numbers of fighters involved and of how they will 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 and the Western- and Saudi-backed exile administration of Ahmad Tumeh.

It is a setback for foreign leaders trying to bolster more secular rebel groups and to reassure voters skeptical of deeper involvement in Syria's civil war. Some may think again about help for the fighters, which includes weaponry from the Gulf as well as non-lethal aid from Europe and the United States.

For Assad, already boosted 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-Qaida.

The most hard-line Islamist militant faction, al-Qaida's Islamic State of Iraq and the Levant, 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 reorganized under an Islamic framework and to be run only by groups fighting inside Syria. Signatories include hard-liners like the Nusra Front and Ahrar al-Sham and 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 recognize them," said the statement read online by Abdulaziz Salameh, the political leader of the Tawheed Brigade.

"Therefore the National Coalition and its supposed government led by Ahmad Tumeh do not represent them and will not be recognized," he said.

Analyst Aron Lund wrote on the blog Syria Comment: "If the statement proves to accurately represent the groups mentioned and they do not immediately fall apart again, it is a very big deal.

"It represents the rebellion of a large part of the 'mainstream FSA' against its purported political leadership, and openly aligns these factions with more hard-line Islamist forces."

Also on Wednesday, the Russian Foreign Ministry said only political and diplomatic methods could settle the crisis in Syria and stabilize the situation in the region as a whole.

During a meeting between Foreign Minister Sergei Lavrov and UN-Arab League envoy to Syria Lakhdar Brahimi, Moscow called for preparations for the international conference on Syria to be completed, the ministry said in a statement.

Reuters-Xinhua