In a continuing spate of violence, four Saudi policemen were killed Tuesday in a shootout with Muslim extremists at a checkpoint northwest of Riyadh, hours after an armed confrontation in the capital left one militant and a security personnel dead, report agencies and local newspapers.

Earlier a police spokesman had said armed extremists fired at a police checkpoint in Qassim, 320 km from the capital,  killing two policemen and wounding four other members of the security forces. The toll was later updated to four.

The assailants then reportedly escaped in a stolen police car. The area has been cordoned and police are conducting house-to-house searches.  

On Monday, an extremist  and a policeman were killed and four securitymen wounded in a shootout in the Al-Faihaa neighbourhood east of Riyadh.

Security personnel also found and defused two car bombs and intercepted another car laden with arms and ammunition.

An American embassy circular asked people to be alert since the US government "continued to receive indications of possible terror attacks" in Saudi Arabia. 

The Saudi authorities have launched a concerted drive against militant outfits following suicide attacks on Western embassies in May and November last year.

On March 15, security forces shot dead Khaled Ali bin Haj, a Yemeni described as the Al Qaeda's head of operations for the Gulf region.