The Isis militant group has seized enough radioactive material from government facilities to suggest it has the capacity to build a large and devastating “dirty” bomb, according to Australian intelligence reports.

Isis declared its ambition to develop weapons of mass destruction in the most recent edition of its propaganda magazine Dabiq, and Indian defence officials have previously warned of the possibility the militants could acquire a nuclear weapon from Pakistan.

According to the Australian foreign minister, Julie Bishop, Nato has expressed deep concerns about the materials seized by Isis from research centres and hospitals that would normally only be available to governments.

The threat of Isis’s radioactive and biological weapons stockpile was so severe that the Australia Group, a 40-nation bloc dedicated to ending the use of chemical weapons, held a session on the subject at its summit in Perth last week.

“This is really worrying them,” Ms Bishop said in an interview with The Australian.

When they swept across territory in Syria and Iraq, she said, “the insurgents did not just clear out the cash from local banks”.

Last week Ms Bishop spoke at the Australia Group meeting about fears Isis was weaponising poisonous gases such as chlorine.

And speaking to The Australian, she confirmed that the concerns she was raising stemmed from reports filed by the Australian department of defence as well the foreign office.

Ancient monuments under Isis threat Show all 8 1 /8 Ancient monuments under Isis threat Ancient monuments under Isis threat Isis seizes Palmyra The ancient oasis city of Palmyra Getty Ancient monuments under Isis threat Isis seizes Palmyra A partial view of the ancient ruins Getty Ancient monuments under Isis threat Isis seizes Palmyra The ancient Palmyra theater Reuters Ancient monuments under Isis threat Isis seizes Palmyra A view of the ancient Syrian city of Palmyra Getty Ancient monuments under Isis threat Isis seizes Palmyra The Temple of Bel Reuters Ancient monuments under Isis threat Isis seizes Palmyra Palmyra's famous graves AP Ancient monuments under Isis threat Isis seizes Palmyra The ancient castle Getty Ancient monuments under Isis threat Isis seizes Palmyra A sculpture depicting a rich family from the ancient Syrian oasis city of Palmyra, displayed at the city's museum Getty

The growing concerns about Isis’s development of weapons of mass destruction come at a time when experts fear the militant group will be “more active than ever” to mark the start of Ramadan and the one-year anniversary of its declaration of a “caliphate”.

Isis said it was changing its name to “Islamic State” following the first public address by its leader Abu Bakr al-Baghdadi in Mosul on 29 June last year, and the US-based Institute for the Study of War has noted that the group usually reserves its major operations to coincide with the Islamic holy month.