Saudi police immediately sealed off the area

The shooting happened in the al-Malaz suburb of the city and police were reportedly seen chasing suspects.

Hours later an Islamist website claimed al-Qaeda militants had kidnapped an American citizen in Riyadh.

A US military contractor and an Irish BBC journalist have also been killed in the last six days amid a spate of attacks on foreigners in the kingdom.

The US embassy in Riyadh reiterated a warning to Americans in Saudi Arabia to leave the country.

'Shot in his car'

Riyadh's police chief told the official Saudi Press Agency that the latest shooting occurred at about 1600 local time (1300 GMT).

12 June - US national gunned down in al-Malaz district 8 June - American working for a US defence contractor killed in al-Khalij district 6 June - BBC cameraman killed and BBC reporter seriously injured in al-Suwaydi suburb 22 May - German chef killed outside a Riyadh bank

"An American resident was shot dead in al-Malaz district," the officer said.

Reports quoted witnesses as saying that the man had been shot as he parked his car in front of his house in al-Malaz district, and that gunmen had moved in to fire more shots.

A US embassy spokesperson said: "We can confirm there was an individual shot who is believed to be an American. We are still investigating."

So far, the victim has not been officially identified.

After the shooting, US Ambassador to Saudi Arabia James Oberwetter said in a statement that "those Americans who choose to remain here should exercise the utmost caution as they go about their daily life".

The Saudi Minister for Islamic Affairs, Saleh bin Abdulaziz al-Shaikh, said that despite the recent attacks, security in the kingdom "has not reached a stage of crisis".

Missing American

A statement on an Islamist website purportedly from al-Qaeda said the group had kidnapped an American in Riyadh on Saturday.

It was accompanied by a passport-sized photograph of a brown-haired man and a business card bearing the name Paul M Johnson, Associated Press news agency reported.

The statement said the hostage would be dealt with in the same way as "the Americans dealt with our brothers in Guantanamo and Abu Ghraib".

Several US soldiers have been charged with abusing Arab prisoners in Abu Ghraib jail near Baghdad.

The US embassy confirmed it was looking for a missing American citizen.

"We do have reports of a missing American. We are working with local authorities to find him and are in touch with his family," an embassy spokesman said.

Recent attacks on foreigners have been blamed on radical Islamists with links to Osama Bin Laden's al-Qaeda network.