The Madain Saleh killings were the first of foreigners since 2004

Saudi sources told the BBC that police were treating the incident as a "terrorist attack".

The shooting happened near the ruins of Madain Saleh, in north-western Saudi Arabia, which is popular with tourists.

Saudi TV said the victims were part of a group of French nationals, some of whom were Muslims heading to Mecca.

But a French diplomat confirmed to the French news agency AFP that none of those shot was Muslim.

Major General Mansour al-Turki, a Saudi interior ministry spokesman, said two men were killed instantly as they rested at the side of the road and came under fire from gunmen.

Another died later in hospital and a fourth was in serious condition, he said.

Some women and children were also part of the group, but were not hurt, he added.

A French diplomatic source, quoted by the French news agency AFP, said an unknown number of attackers "machine-gunned them while they got out [of their vehicle] to go for a walk".

Violent campaign

French Foreign Minister Philippe Douste-Blazy said in a statement that "several of our compatriots residing in Riyadh were the victims of an armed attack".

The French President Jacques Chirac described the attack as "hateful".

SAUDI ATTACKS ON FOREIGNERS May 2003: Suicide bombers in Riyadh kill 35 people May 2004: 22 civilians die in Khobar compound shootout June 2004: Gunmen kill BBC cameraman Simon Cumbers and seriously injure correspondent Frank Gardner June 2004: US contractor Paul Johnson beheaded No fatal attacks since 2004

"I condemn with the utmost strength this horrible act," he said, promising to lend all support to the Saudi authorities in bringing the perpetrators to justice.

BBC security correspondent Frank Gardner says the attack will come as a blow to the Saudi government, which has been fighting a successful campaign against Islamist militants since they launched a triple truck bombing in 2003.

A number of foreigners have been killed - including Frenchman Laurent Barbot, who was shot dead in the Red Sea port city of Jeddah by suspected al-Qaeda militants in September 2004.

But there have been no foreigners killed since late 2004.