A group led by al-Qaida ally Abu Mussab al-Zarqawi said on Wednesday that it will attack US-run prisons in Iraq in retaliation for the killing of four inmates two days earlier, according to a statement posted on the Internet.

"What's the crime of these unarmed prisoners? We'll punish you (US), God willing," said the statement.

Vowing to respond decisively, the group told the detainees "to be patient," saying "the victory is near."

The authenticity of the statement could not be verified independently, but the al-Qaida linked network often publishes announcements and videos on the website.

The US military said four Iraqis were killed and six injured in a riot at the Camp Bucca confinement in the Iraqi southern Um Qaser area on Monday.

The violence erupted after a routine search for contraband in one of the camp's 10 compounds, the military said in a statement.

Three of the six injured were evacuated for treatment at a military hospital, while the remaining three were treated at the facility's medical clinic.

Human rights groups have cautioned that US captors who fired at the inmates could have violated the international convention.

The US military said the incident was under investigation.