"Hurrying to kill this wanton tyrant (King Abdullah) who has announced himself to be a leader of atheism would be among the most pious acts," Abu Yahya al-Libi said in a video posted on Monday on Islamist websites often used by al Qaeda supporters.

The message followed a groundbreaking Saudi-sponsored interfaith conference in Spain this month in which King Abdullah called on followers of the world's major faiths to turn away from extremism and seek reconciliation.

"The call for a rapprochement of religions issued by the (Saudi) tyrant ... is an integral part of the overt Crusader war against Islam and Muslims," Libi said. He likened King Abdullah to a "corrupting fountainhead" that had to be buried.

"This in fact is a call to turn one's back on Islam and ... to look for commonalities with Judaism and Christianity so whatever the three agree on would become the new modern religion which would be allowed to be propagated," Libi said.

Libi frequently issues messages on behalf of al Qaeda which appears to be grooming him as a top group spokesman since he escaped from a U.S. jail in Afghanistan in 2005.

The Madrid gathering aimed to draw Muslims, Jews and Christians closer together and isolate those who use religion to justify violence or intolerance. It was the first time Saudi Arabia, where non-Muslims cannot practise their faith openly, had invited Jews to such a meeting.

The king also invited Buddhists, Hindus and Sikhs as he showcased a more tolerant side to Saudi Arabia's strict Wahhabi Islam, under fire since 15 Saudis were among the 19 Arabs who carried out the September 11, 2001 attacks in New York.