ABU DHABI ø Al Qaida has expanded its insurgency campaign to target oil assets in Saudi Arabia.

At least four Al Qaida gunmen infiltrated an oil refinery and petrochemical complex in the Saudi city of Yanbu and killed at least five Westerners. The attackers opened fire from M-16 and AK-47 assault rifles and also injured 20 others in the complex on Saturday morning.

This was the first time Al Qaida targeted a Saudi energy facility. Western intelligence sources said that over the last six months Al Qaida was planning to attack the kingdom's oil facilities as part of an effort to endanger the royal family. Yanbu is the largest oil port along the Red Sea.

Saudi officials acknowledged that Al Qaida has infiltrated the Saudi work force at the Yanbu oil facility, Middle East Newsline reported. They said three of the four attackers were employees at the refinery and petrochemical complex.

"Workers at a petrochemical site used their passes to access the complex and carry out the attack," a Saudi Interior Ministry official said. "Three employees entered the site, taking advantage of their passes and allowing a fourth accomplice to enter through the emergency gate."

Two of the casualties in the Al Qaida attack were U.S. nationals. Two British nationals and an Australian, all of whom were identified as engineers, were also killed in the shooting.

"The kind of horrific attacks that happened today in Yanbu must be condemned by all those who want to see peace and prosperity in the Middle East," U.S. ambassador James Oberwetter said in a statement.

The Yanbu refinery, located 350 kilometers north of Jedda, is owned by ExxonMobil and the Saudi company SABIC. The kingdom-owned Saudi Aramco ø responsible for Saudi oil production ø issued a pledge to protect its facilities from attack.

The attackers, described as teenagers, were said to have stripped naked one of the Westerners in the refinery and tied him to a get-away vehicle.

The man was dragged along the road for about three kilometers as police and security forces gave chase. In the ensuing gunbattle, at least three of the insurgents as well a Saudi National Guard officer were killed.

The Westerners were employees of ABB Lummus, a Texas-based subsidiary of the Swiss-Swedish engineering and oil services firm ABB. Executives said the company was considering withdrawing its personnel from Saudi Arabia.

In another part of the city, Saudi police battled two Islamic insurgents who had commandeered a coast guard patrol jeep in Yanbu. One of the insurgents, who also wore coast guard uniforms, was killed and found wearing an explosive belt.