Key Syrian rebel groups, including members of the Western-backed Free Syrian Army, have rejected the authority of the Turkey-based Syrian National Coalition, a body that the West and some Arab countries hoped would form a government in exile.

The coalition this month elected self-described moderate Islamist Ahmad Tumeh to serve as the prime minister of its government, but 13 of the most powerful rebel groups fighting in Syria said neither him nor the coalition represent them.

The majority of the rebels denouncing the SNC are radical Islamists such as the notorious terrorist organization Al-Nusra Front. But among them is the 19th Division, a significant but relatively new addition to the mainstream Free Syrian Army, reports AFP.

"These forces call on all military and civilian forces to unite under a clear Islamic framework based on Sharia law [Islamic law], which should be the sole source of legislation," the signatories said.

The statement was published on several rebel-related social media pages. It was read by an elderly man shown on a video.

Radical Islamists and their more moderate fellow fighters had previously banded together on numerous occasions for joint operations against the forces of Syrian President Bashar Assad.

Some groups from the Free Syrian Army receive non-lethal support from the US and other Western countries. Washington says it may supply weapons as well, pledging that it can vet the recipients to ensure that the arms will not get into radical Islamists’ hands.

Members of Syrian National coalition (SNC) attend a meeting of the National Coalition of Syrian Revolution and Opposition forces in Istanbul (AFP Photo / Ozan Kose)

 
Meanwhile a senior US State Department official said Tuesday that radical and moderate rebels in Syria are engaged in the fiercest infighting yet, especially in opposition-held territory along Syria's northern and eastern borders. The increased confrontation between the more effective Islamists and the US-backed moderates has undermined their ability to fight against the Syrian army.

"It's the hardest fighting we have ever seen between Salim Idriss's elements of the Free Syrian Army and the Islamic State of Iraq and the Levant," said the official, who spoke to reporters after US Secretary of State John Kerry met SNC President Ahmad Jarba. "It's a slog."

"I would even go so far as to say that the extremists are actually doing the government's work now, which was a point that the opposition made in the meeting with the secretary," said the official, who spoke on condition of anonymity.

A recent study by defense consultancy IHS Jane's said that more than half of Syrian rebels currently on the ground are jihadists, many of them foreign fighters. The influence of the moderate armed opposition has been dwindling for months, as they failed to form efficient command structures and lost ground to radical forces. The Islamists can employ militant combat experience gained in militant battlefields outside of Syria and are considered to have higher moral authority.

The denouncement came just days after the SNC announced that they may attend the Geneva-2 peace conference, an international event, which may become a peaceful political solution to the Syrian conflict. Prior to that, all Syrian opposition groups had refused to engage in any talks with the Assad government, a position which caused organizers of the conference to postpone it several times.

Both Western backers of the Syrian rebels and countries such as Russia, China and Iran, which side with the Syrian government, see gathering the conference as the preferred development in the two-and-a-half-year armed conflict, which has already claimed over 100,000 lives according to UN estimates.

• A Permanent Economic Emergency