An official inquiry has been opened into the slaughter, with investigators trying to determine whether the army's ranks had been infiltrated by the insurgents.

"An armed terrorist group killed 49 unarmed soldiers on leave heading to their families along with three other civilian drivers and burned three vehicles," the defence ministry said in a statement.

"These cowardly and treacherous actions will not stop our fighters of the courageous army from carrying on to purify the soil of Iraq from the criminal and terrorism groups."

The ambush took place late Saturday as the cadets drove in minibuses towards the south of Iraq from a training base near the eastern border with Iran. It was one of the bloodiest attacks against Iraq's security forces, a favoured target of insurgents fighting US-led forces and the US-backed government.

"As we condemn this criminal action, we promise our patient people to make the criminal terrorists get the fair punishment for their crimes against our people," the ministry said.

Accusations were flying in Iraq about how the new recruits had been left defenceless in the windswept desert plain, cut off from all forms of communication.

The newly-trained soldiers "were sent on the road without any weapons," said Naim al-Kaabi, an aide to radical Shiite cleric Moqtada al-Sadr, a fierce opponent of interim Prime Minister Iyad Allawi.

Nassir al-Shadershi, a member of Iraq's interim parliament, accused the authorities of failing in "their obligation to protect our own security forces".

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