The Islamic State militant group (ISIS), on the back foot in Iraq and Syria, is bringing together its top chemical weapons experts in the border area between the two countries with the aim of creating an elite “chemical weapons cell,” U.S. intelligence believes.

The cell, according to a U.S. official speaking to CNN, consists of the jihadi group’s most senior specialists from the territory it controls in Iraq and Syria.

The U.S.-led coalition, responding to a Newsweek request, said it was “looking into” the disclosure by the U.S. official.

A coalition of Kurdish-led militiamen are waging a ground offensive, with the air support of the U.S.-led coalition, on the ISIS-held eastern Syrian city of Raqqa. U.S. officials and experts on the group have long held Raqqa to be the de-facto capital of its self-declared caliphate. But the official said that assessment is changing within U.S. intelligence circles.

The advance on Raqqa

Under pressure, the group has shifted its assets from Raqqa to the Euphrates River Valley region, between Mayadin in Syria, and the town of al Qaim, an Iraqi border town. The official said the barren border area is now considered the de-facto capital of ISIS territory, with potentially thousands of ISIS militants there.

It is the same region where Iraqi and U.S. officials believe the most wanted terrorist in the world, ISIS leader Abu Bakr al-Baghdadi, could be in hiding, moving across the Iraqi-Syrian border areas that the group controls.

“He moves between al-Hajin in Syria and al-Ba'aj in Iraq,” the official told Newsweek in March. Officials believe the self-declared caliph of the world’s Muslims fled the northern Iraqi city of Mosul ahead of a large-scale ground offensive by Iraqi forces, now in its seventh month.

Raqqa still represents a key capture for the U.S.-led coalition as it has been the largest Syrian city in the group’s possession since the beginning of 2014. As the group continues to lose territory in Iraq and Syria, it has mounted a staunch defense of its strongholds with sniper fire, booby traps and suicide car bombs. But the development of chemical weapons capabilities could pose a greater threat to advancing forces. Even so, the militant group has already used such weapons against enemy forces on several occasions.

In November, defense consultancy IHS Janes reported ISIS had used chemical weapons against enemy forces at least 52 times since its rise to global prominence from January 2014 onward. Nineteen of those attacks were launched in and around the Iraqi city of Mosul.

The U.S.-led coalition continues to focus its efforts on targeting the group’s chemical weapons capabilities. In September, the Pentagon said it had destroyed a complex of buildings in northern Iraq that housed an ISIS chemical weapons factory, and in so doing had removed a “significant chemical threat” to civilians.