Saudi Arabia's de facto leader vowed to use an iron fist to crush Islamic extremists after Saturday's attack on a petrochemical plant in the Red Sea port which killed two Americans, two Britons, an Australian and a Saudi national.

Four of the attackers and one security agent were also killed in a shootout. One of the assailants was reported to be Abdullah Saud Abu Nayan As-Subayi who is on a list of the kingdom's 26 most wanted terror suspects.

Swiss-Swedish engineering company ABB, who employed the slain Westerners, said it is evacuating all international staff and their families from Yanbu.

"I saw three armed men pull (a victim) close to my school and shout 'Help your brothers in Fallujah (Iraq). Jihad! Jihad! God is great!" said a local student. Another witness added: "The American was mutilated."

The English-language Saudi Gazette reported on its website on Sunday that "a Westerner was kidnapped, stripped naked, tied to a vehicle, and dragged along the road as the attackers made their getaway".

It also ran a photograph of a mutilated body lying in a street before a crowd of onlookers.

Crown Prince Abdullah bin Abdul Aziz, the kingdom's de facto ruler, vowed that the Saudi authorities would crush terrorists, who he said were manipulated by "Zionism."

"Your country is targeted... Zionism is behind what is happening. This has become clear -- I wouldn't say 100 percent but 95 percent," SPA quoted him as saying on Saturday.

"We will strike with an iron fist at anyone who undermines the security of this country. We will pursue this deviant group for 20 or 30 years."