BEIRUT: Syria's army has been pounding major bases of the Islamic State of Iraq and Greater Syria in coordination with the Baghdad government for the last 24 hours, an activist group says Sunday.

The strikes against ISIS -- 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 ISIS'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 Assad was responding to the fact that ISIS "brought into Syria heavy weapons including tanks" captured from the Iraqi army.

In Raqa, the air force bombed the area surrounding ISIS'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. Saturday, the regime also bombarded ISIS'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 ISIS, 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 ISIS in areas where it and other Islamist militants have advanced in northern Iraq in the past week.

ISIS 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 ISIS 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 ISIS has killed more than 6,000 people, mostly fighters, since it broke out in January.