A huge blast on Friday rocked the city of Idlib, a stronghold of an al-Qaida branch in northwestern Syria, killing at least 11 people, a war monitor reported.

The Syrian Observatory for Human Rights added that the cause of the explosion was not immediately clear.

Locals in Idlib said the blast had rocked the headquarters of the al-Qaeda-linked Hayat Tahrir al-Sham (HTS), which controls the area.

One source, who requested anonymity citing security concerns, said the blast was caused by a car bombing that targeted the HTS headquarters in the city.

Seven of the 11 fatalities were fighters, Observatory head Rami Abdel-Rahman said.

Earlier this month, the al-Qaida's affiliate in Syria extended its control over an enclave previously held by the moderate National Liberation Front (NLF), after both sides reached a cease-fire deal ending their nine-day fight there.

Idlib is the capital city of the province of the same name, which is the last key opposition stronghold in Syria.

In September, Turkey and Russia reached a deal to establish a demilitarized buffer zone in the enclave, a move that prevented a major regime offensive there.