Islamic State group has reportedly developed a nuclear weapon made from radioactive material stolen from an Iraqi university, according to a militant who claims insider knowledge. Hamayun Tariq, a British ISIS member now based in Syria, claimed on social media that the group obtained the uranium from Mosul University and now possesses a “dirty bomb” that it is now considering detonating in a public area.

If true, this would confirm fears voiced by Iraq’s United Nations ambassador back in July following the seizure of 40 kilograms of uranium compounds from Mosul University. In a letter to U.N. Secretary-General Ban Ki-moon dated July 8, ambassador Mohamed Ali Alhakim warned that these materials “can be used in manufacturing weapons of mass destruction,” according to Reuters. "These nuclear materials, despite the limited amounts mentioned, can enable terrorist groups, with the availability of the required expertise, to use it separate or in combination with other materials in its terrorist acts," said Alhakim.

The claims by Tariq, who now goes by the nom de guerre Muslim al-Britani, were first reported by the U.K. newspaper the Mirror, which also reported that militants were boasting about the damage such a weapon could cause if detonated in London. Tariq has reportedly had his British passport canceled by the U.K. Home Office, according to the Mirror.

Nuclear experts, however, have cast doubts about the danger posed by the stolen radioactive material. The uranium that Islamic State is reportedly in possession of likely poses more of a danger as a toxin, former U.N. nuclear weapons inspector Bob Kelly told NBC. The U.N. nuclear agency has similarly played down the threat, saying that the material ISIS likely possessed was “low-grade” and did not pose a major threat, according to NBC. It is also unlikely that ISIS would be capable of transporting a nuclear weapon, if it existed, outside of Syria or Iraq, reported the Mirror.

The issue of Islamic State possessing nuclear weapons would have implications for the conflict in Syria as President Barack Obama has specifically designated the scenario as one that would necessitate the involvement of U.S. ground troops. U.S. officials, however, have maintained that there is no indication that ISIS could easily obtain such a weapon, according to ABC.