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A hardline Islamist rebel group in Syria has called on its supporters to attack rival opposition factions that do not support its cause, the BBC reported.

The call by the al-Qaeda-linked Islamic State in Iraq and the Levant (ISIS) follows days of rebel infighting that has left scores dead.

It came hours after another rebel group called on ISIS to observe a ceasefire.

The Nusra Front said the factional fighting benefited the government of Syrian President Bashar al-Assad.

Opposition activists say some 270 people have been killed in factional fighting since Friday. Clashes are continuing in Raqqa, a city under full rebel control and previously an ISIS stronghold.

BBC world affairs correspondent Mike Wooldridge says the damage being done to forces opposing President Assad was clearly recognised in an audio message from Nusra Front chief Abu Mohammed al-Golani, in which he called for a truce.

"The regime will gain new life when it was close to collapse," he said in an audio message posted on Twitter.

"(The fighting) risks costing us dearly on the ground if it continues."

He accused ISIS - also known as ISIL - of having a "flawed policy" that had played "a key role in fuelling the conflict."

The Nusra Front leader proposed an initiative to end the fighting that would include a ceasefire, a prisoner exchange and setting up an Islamic committee to mediate disputes.

However, a defiant ISIS audio message later urged its fighters to attack other rebel groups.

Spokesman Abu Mohammed al-Adnani - although making no reference to the Nusra Front initiative - called on the militants to "crush them (the rebels) totally and kill the conspiracy at birth."

He also threatened all members of the main opposition grouping, the Syrian National Coalition (SNC).

"Everyone who belongs to this entity is a legitimate target for us, in all places, unless he publicly declares his rejection of that group and of fighting the mujahideen," he said.