Fighters formerly linked to al-Qaeda took control of Idlib in Syria on Sunday after rival rebels withdrew, strengthening their grip on the northwestern city and its province, one of the last beyond regime control.

At the same time a car bomb exploded in Idlib killing 11 people, the UK-based Syrian Observatory for Human Rights monitoring group said.

The latest developments come after the armed group Hay'et Tahrir al-Sham, which is dominated by a former al-Qaeda affiliate and known as HTS, agreed a ceasefire on Friday with Ahrar al-Sham rebels.

"Ahrar al-Sham withdrew from the city of Idlib which is now under the control of Hay'et Tahrir al-Sham," Observatory head Rami Abdel Rahman told AFP news agency.

READ MORE: Syria's civil war explained from the beginning

"Hundreds of rebels left the city aboard dozens of vehicles heading towards southern Idlib province," he said.

The truce ended a week of fierce fighting between HTS and Ahrar al-Sham, which is backed by Turkey and some Gulf countries, that killed at least 92 people including 15 civilians, the Observatory said.

Abdel Rahman said the HTS set up checkpoints across the northwestern city.

The fall of the city and provincial capital is symbolic.

It comes after HTS captured in a bloodless takeover "more than 31 towns and villages" across Idlib province over the past two days, the monitor said.

Former allies

The HTS is dominated by the Fateh al-Sham faction, which was previously known as al-Nusra Front before renouncing its ties to al-Qaeda.

The HTS and Ahrar al-Sham were once allies and fought alongside each other to capture most of Idlib province from Syrian government forces in 2015.

The truce they agreed to on Friday calls for the release by both sides of prisoners and the "withdrawal of armed groups from the Bab al-Hawa" border crossing with Turkey.

Bab al-Hawa, which had been controlled by Ahrar al-Sham, would be handed over to civilian administration, it said.

Abdel Rahman said the presence of Ahrar al-Sham rebels had been greatly diminished in Idlib province, which they once dominated. Rebels were left only in Ariha town and part of Jabal al-Zawiya in the southeast.

Ahrar al-Sham has also been hit by the defection of hundreds of its fighters to HTS.

Syria's conflict erupted in mid-March 2011 with peaceful anti-government protests that were brutally repressed by the government.

It quickly evolved into a war involving local, regional and international players on a multitude of fronts, which has killed more than 330,00 people and displaced millions from their homes.