Syria's army has been pounding for 24 hours major bases of the Islamic State of Iraq and the Levant in coordination with the Baghdad government, a monitor said Sunday.

The strikes against ISIL -- which has spearheaded a week-long jihadist offensive in Iraq -- have been more intense than ever, said the Syrian Observatory for Human Rights.

"The regime air force has been pounding ISIL's bases, including those in the northern province of Raqa and Hasakeh in the northeast," which borders Iraq, said the Britain-based group.

The regime of President Bashar al-Assad was responding to the fact that ISIL "brought into Syria heavy weapons including tanks" captured from the Iraqi army.

In Raqa, the air force bombed the area surrounding ISIL's main headquarters in Syria, as well as the group's religious courts, said the Observatory, adding there were no reported casualties.

Photographs sent by activists in Raqa that could not be independently verified showed craters in the ground and rubble in front of the main gates of the headquarters, a former town hall.

On Saturday, the regime also bombarded ISIL's headquarters at Shaddadi in Hasakeh, home to a frontier crossing from Iraq that is under the jihadists' control.

Observatory director Rami Abdel Rahman said the strikes were the regime's most "intense" against ISIL, and that they were being carried out "in coordination with the Iraqi authorities".

The government in Baghdad has been gearing up for a counter-offensive against ISIL in areas where it and other Islamist militants have advanced in northern Iraq in the past week.

ISIL espouses a radical interpretation of Islam, and aims to set up a state stretching across the Syria-Iraq border. It has been accused of committing widespread human rights abuses in Syria.

Once welcomed in Syria by rebels seeking Assad's overthrow, the well-armed and well-organised ISIL soon gained the Syrian opposition's wrath because of its quest for hegemony and systematic abuses.

In 2013, it took part in operations against government forces. But in recent months, it has exclusively fought against the Syrian rebels, who accuse the group of serving the interests of Assad's regime.

A war pitting Syrian rebels against ISIL has killed more than 6,000 people, mostly fighters, since it broke out in January.