The bodies of almost 600 victims from one of the worst atrocities ever committed by the Isis militant group have been exhumed from sites in northern Iraq, officials have said.

Speaking at a press conference in Baghdad, Iraq's human rights minister Mohammed al-Bayati gave the latest update on a body count that is expected to continue to rise – possibly even double.

The bodies are all those of young air cadets, massacred after the fall of the northern city of Tikrit to Isis almost exactly one year ago.

Tikrit, famous for being the home town of Saddam Hussein, fell to the so-called “Islamic State” on 12 June 2014, two days after the larger city of Mosul, now the group's biggest stronghold in Iraq.

According to reports, images and videos posted online by ISIS at the time, the group also captured the Speicher military base near Tikrit, and with it around 4,000 unarmed air force recruits.

Between 1,000 and 1,700 mostly Shia recruits were executed and buried in mass graves in several locations.

Tikrit was recaptured by government forces in April, the start of a mammoth task for the state's forensic scientists to identify those massacred.

“The remains of 597 Speicher martyrs have been exhumed,” Mohammed al-Bayati told journalists in Baghdad.

In a previous update, health minister Adila Hammoud described the work to exhume the Speicher victims as “complicated”.

“There were several layers of bodies all piled on top of each other,” he said. “It's a huge case. It takes a lot of work to identify the victims.”

'ISIS closes in on to get dirty bomb'

In another worrying development, an Australian intelligence reports said the militant group has seized enough radioactive material from government facilities to suggest it has the capacity to build a large and devastating "dirty" bomb, . 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.

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

She confirmed that the concerns she was raising stemmed from reports filed by the Australian department of defence as well the foreign office.

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".