A roadside bomb blast south of the Iraqi capital has struck the convoy of a private security firm, killing a Briton and two Iraqi guards.

The convoy was struck just outside the city of Hilla Thursday night, AFP quoted a US military statement as saying.

"The car was completely destroyed in the attack in the Nile district, north of Hilla," AFP quoted Iraqi army's Lieutenant Karim Qasim as saying on Friday.

The military and the company employing the guards both declined details on the convoy and the identity of the blast victims.