Jihadists AFP photo

The Western-backed Syrian National Coalition (SNC) on Wednesday accused an Al-Qaeda-linked group in the country of ties to the Syrian regime, AFP reports.

The criticism is the strongest yet by the SNC of the Islamic State of Iraq and the Levant (ISIL), and comes after the group reportedly tortured and killed an opposition doctor in northern Syria.

"The Coalition believes that ISIL is closely linked to the terrorist regime and serves the interests of the clique of President Bashar Al-Assad, directly or indirectly," it said in a statement quoted by AFP.

"The murder of Syrians by this group leaves no doubt about the intentions behind their creation, their objectives and the agendas they serve, which is confirmed by the nature of their terrorist actions hostile to the Syrian revolution," it added.

The statement called on rebels who had joined ISIL to abandon the group and for the "prosecution of the leaders of this terrorist organization along with the criminals of the regime."

The Coalition said an opposition doctor, Hussein al-Sleiman, known by the pseudonym Abu Rayyan, had been held by ISIL in Maskana in Aleppo province, though there were no details on when he was detained.

The statement said he had been shot dead after being subjected to "the worst forms of torture."

The Syrian Observatory for Human Rights said the doctor belonged to the Ahrar al-Sham rebel group and that "one of his ears was cut off before he was shot."

"His body was handed over on Tuesday as part of a prisoner exchange," the NGO said, according to AFP.

ISIL is one of several jihadist rebel groups operating in Syria and which have carried out atrocities during the civil war, such as the public beheading of a Catholic priest who was accused of collaborating with Assad’s regime.

Executions of members of rival rebel factions have become a regular occurrence in war-torn Syria.

A 13-member Islamic Front for the Liberation of Syria at one point split off from the Western-backed Syrian National Council opposition force and declared Aleppo to be an independent Islamist state.

Since that time, a second civil war has begun in war-ravaged Syria and now, in addition to fighting the Syrian army, the more moderate rebel groups and the Islamist extremist groups are also fighting each other.

The SNC, noted AFP, has in the past accused the regime of having helped to set up extremist groups among the opposition in a bid to "steal" the revolution, without naming ISIL.

It has also accused the group of abandoning the fight against the regime and instead battling other opposition fighters and civilians.

The Observatory said ISIL is fighting in the area around Damascus and the north, though it has also been accused of fighting rebels and committing abuses against civilians.