Al Qaeda-linked jihadists struck back against rival rebels in eastern and northern Syria on Friday, Jan 10, after a week of internecine fighting among opponents of President Bashar al-Assad in which 500 people have been killed, a monitoring group said, according to Reuters.

A coordinated offensive by armed groups had seized several strongholds of the Islamic State in Iraq and the Levant in Aleppo, on the border with Turkey, and further east in Raqqa - the only city under control of Assad's foes.

But the Syrian Observatory for Human Rights said ISIL fighters pushed back rival rebels on the eastern approaches of Raqqa on Friday. They also killed 20 fighters in the town of Al-Bab, north-east of Aleppo, the monitoring group said.

The fighting comes less than two weeks before the planned start of international peace talks aimed at ending nearly three years of conflict in Syria, which the Observatory says has killed more than 130,000 people.

Most rebels are opposed to the negotiations, known as Geneva 2, and the main opposition group in exile, the National Coalition, has delayed a final decision on attending the talks until just days before their January 22 start date.

French Foreign Minister Laurent Fabius said on Thursday he was not sure the conference would take place. Syria's Muslim Brotherhood said conditions were not right to hold it because international powers had not done enough to ensure its success.

In a statement on Friday, the Brotherhood, which has members in the coalition, set out conditions for attending including release of detainees, opening humanitarian corridors to besieged areas and the withdrawal from Syria of Iranian, Iraqi and Hezbollah fighters who back Assad.

Opposition figures from another coalition group, the Syrian National Council, have already said they will shun Geneva because world powers have not done enough to force Assad to step aside.