Syria's opposition National Coalition on Wednesday accused an Al-Qaeda-linked group in the country of ties to the Syrian regime, saying it was serving the government's interests.
The criticism is the strongest yet by the coalition 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.
"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.
It called on rebels who had joined ISIL to abandon the group and for the "prosecution of the leaders of this terrorist organisation 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".
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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.
The Coalition 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.
Syria's conflict began with peaceful anti-government demonstrations in March 2011, and spiralled into a war after a brutal regime crackdown.
Initially, the rag-tag armed opposition welcomed the arrival of jihadist fighters, who were mostly better-equipped and trained, but relations soured with the increasing strength of the foreign groups.
The Syrian regime has always branded all those fighting against it as "terrorists".