Jihadist fighters linked to Al-Qaeda set fire to statues and crosses inside churches in northern Syria on Thursday and destroyed a cross atop the clock tower of one of them, a human rights watchdog said.

Fighters from the Islamic State of Iraq and the Levant (ISIL) entered the Greek Catholic Church of Our Lady of the Annunciation in the northern city of Raqa and torched the religious furnishings inside, the Syrian Observatory for Human Rights said.

They did the same at the Armenian Catholic Church of the Martyrs, also destroying a cross atop its clock tower and replacing it with the ISIL flag, the Observatory said.

Most of Raqa, located on the banks of the Euphrates River and capital of the province of the same name, fell to rebels in March. Where it dominates in the city, ISIL imposes a strict version of sharia (Islamic law) on the populace.

The London-based Observatory denounced these attacks as "against the freedom of religion, which is an assault on the Syrian revolution."