The Iraqi ministry of science and technology informed the IAEA of the disappearance of 342 tonnes of mainly HMX and RDX explosives on 10 October, agency spokeswoman Melissa Fleming said on Monday.
She said the substances could be used in a nuclear explosion device as the blast to trigger the chain reaction and that is why it was being monitored by the IAEA before the March 2003 US-led invasion of Iraq.
The New York Times reported that the explosives could produce bombs strong enough to destroy aeroplanes or tear buildings apart.
Fleming said: "From a proliferation standpoint, there is a possible application in nuclear weapons. The most immediate concern is the threat of the explosive falling in the wrong hands and being used to commit terrorist acts," she said.
The ammunition dump was
under US military control
According to The New York Times, the explosives disappeared from the sprawling al-Qaqaa facility, which "was supposed to be under American military control but is now a no man's land, still picked over by looters as recently as Sunday".
The report added: "White House and Pentagon officials acknowledge that the explosives vanished some time after the American-led invasion."
After UN weapons inspectors left Iraq under US pressure last year the IAEA's ability to monitor was impaired.
"Our only ability to monitor these sites is through satellite imagery," said Fleming. "It was very difficult to detect here [from IAEA headquarters in Vienna], because the things were in bunkers."
White House spokesman Scott McClellan said US President George Bush had been informed of the disappearance from an unguarded military installation.
"White House and Pentagon officials acknowledge that the explosives vanished some time after the American-led invasion"
New York Times report
The explosives went missing "because of some looting that went on in Iraq towards the end of Operation Iraqi Freedom, or during and towards the end of Operation Iraqi Freedom," McClellan said.
IAEA chief Muhammad al-Baradai has repeatedly voiced concern about the disappearance in Iraq of so-called duel use nuclear material, which could be used for either conventional or nuclear means.