Baghdad  - In a major crackdown Tuesday, Iraqi special forces backed by US helicopters raided Diyala provincial government headquarters and clashed with security guards working for Diyala's governor, security officials said.

Governor Raad Rasheed al-Mulla's secretary, Abbas al-Tamimi, was killed in the operation which took place in the provincial capital Baquba, sources said.

The forces also arrested a number of Diyala council members including Hussein al-Zubeidi, the official in charge of the council's security committee in central Baquba.

As part of the crackdown, US-led forces south-west of Baquba also arrested Diyala University President Nizar al-Khazraji, clashing with the university security guards and killing four.

According to Deutsche Presse-Agentur dpa sources, the operation started at dawn and lasted for four hours. It was not immediately clear what prompted the crackdown.

According to Iraqi security officials speaking to dpa, the Iraqi government had been "kept in the dark" about the crackdown and any Iraqi special units were being led by US forces.

Iraqi Prime Minister Nuri al-Maliki has formed a committee to investigate the incident, Iraq's national TV channel reported.

In other news, a civilian was killed in a car bomb blast in southern Baghdad Tuesday, while four people were injured when a roadside bomb targeting a Kurdish security patrol exploded in Iraq's northern Mosul province, a news report said.

The car bomb had been placed under a civilian's vehicle in Baghdad's central Zaafaraniya district and went off when he entered the vehicle, an anonymous police source told the Voices of Iraq (VOI) news agency.

Another bomb exploded on a roadside in central Talkeef in Mosul province, hitting a passing Kurdish security agency patrol. Four people, including two Kurdish security men, were wounded, VOI reported.

Mosul is located some 400 kilometres north of Baghdad. (dpa)