Risha, a key Sunni Arab ally of the United States and the Iraq government, was killed when a powerful bomb hit his envoy on Thursday. The attack also killed three of his body guards, said a report by the BBC.

The report adds that Risha, who ran a construction and import-export business with offices in Jordan and Dubai, was among a group of tribal leaders who met US President George W Bush [Images] during his visit to Iraq last week. The pro-US Sunni sheikh was assassinated hours before Bush announced his much-awaited decision about partial withdrawal of American troops from Iraq.

Iraq prime minister Nuri al-Maliki blamed al-Qaeda for the attack. Risha formed the Anbar Awakening Conference, a coalition of 42 Sunni tribes, last year. According to the BBC, the conference defeated the Al-Qaeda in western Iraq and this made Risha a top target for the al-Qaeda.

The assassination also coincided with the first day of the Muslim holy month of Ramadan. The Iraq government termed Risha's death as a 'national disaster' and hailed him as a 'national hero'.  The White House called his assassination an 'outrage'.

The incident threatens to unleash a series of retaliatory attacks by the Sunnis, who have vowed revenge against the al-Qaeda.