By Frank Gardner BBC security correspondent in Riyadh

Most Saudis were appalled by the recent hostage slaughter in Khobar

The move follows last weekend's raid by al-Qaeda militants on the Gulf oil town of Khobar which left 22 people dead.

The edict "urges citizens and (foreign) residents to inform on anyone planning... an act of sabotage", reported the official Saudi news agency.

The edict, or fatwa, was issued by a government-backed committee.

The agency said the committee, headed by the Grand Mufti Sheikh Abdul-Aziz al-Sheikh, condemned the recent attacks blamed on al-Qaeda.

It quoted the committee as urging extremists to "fear God almighty and come to their senses".

It is a sentiment likely to be shared by many in this normally peaceful country.

Most Saudis are appalled by the violent attacks on both national and western targets that have killed over 80 people in the last 12 months.

Al-Qaeda threat

But a group calling itself the al-Qaeda Organisation in the Arabian Peninsula continues to threaten more attacks and several of its members are known to be at large.

On Wednesday, a statement in its name claimed responsibility for one of the most recent attacks - that on US military advisers who were shot at while they were driving south of Riyadh.

Osama Bin Laden's original aim was to drive US forces out of Saudi Arabia, but since all but a handful left last year, al-Qaeda's aims here have broadened into attacking Western civilians and government targets.