Suicide bombers in explosives-packed cars attacked the world's largest oil processing facility Friday but were stopped outside the gates when guards opened fire, detonating their vehicles, officials said.

Guards began shooting when two cars tried to drive into the heavily protected facility in eastern Saudi Arabia, Interior Ministry spokesman Lt. Gen. Mansour al-Turki told The Associated Press. Both vehicles exploded, the attackers were killed and two guards were critically wounded, al-Turki said.

Saudi Arabia's oil minister said the blast "did not affect operations" at the Abqaiq facility, but oil prices spiked on world markets.

Crude-oil futures jumped by more than $2 a barrel after the attack rattled an oil market already jittery about supply disruptions in Nigeria and a diplomatic standoff over Iran's nuclear ambitions

Light sweet crude for April delivery rose $2.16, to $62.70 a barrel, on the New York Mercantile Exchange. Brent crude futures for April jumped $1.96 to $62.50 on London's ICE Futures exchange.

It was the first attack on an oil facility in Saudi Arabia and it targeted one of the most important. The huge processing facility near the Persian Gulf coast handles around two-thirds of the country's oil output, according to the U.S. Department of Energy's Energy Information Administration.

The facility "continued to operate normally. Export operations continued in full," minister Ali Naimi, said in a statement.

Initial reports had said the attack briefly halted the flow of oil.

The attack took place in a region where Saudi Arabia's Shiite minority is centered, amid an uproar over the bombing of a major Shiite shrine in Iraq this week.

But suspicions quickly fell on Al Qaeda-linked militants, raising fears of a new tactic emulating Iraqi insurgents, who have hobbled their country's oil industry with sabotage and attacks.

"In Iraq they zeroed in on oil and this appears to be a creeping process, since it is happening in Saudi Arabia," said Youssef Ibrahim, a Dubai-based political risk analyst with the Strategic Energy Investment Group.

Saudi Arabia has been waging a fierce three-year crackdown on Al Qaeda militants, who launched a campaign in 2003 aimed at overthrowing the royal family with a string of attacks mostly targeting foreigners.

Al-Naimi, the oil minister, said "security forces and Aramco security officials managed to thwart a terrorist attack against" the installation. He said the attack caused "a small fire" that was brought under control and did not affect operations.

Saudi-owned Al-Arabiya satellite television reported that the attackers' cars bore the logo of Aramco, the state oil company that owns the facility.

A Saudi journalist who arrived at the scene soon after an explosion said guards exchanged fire for two hours with two militants outside the facility. He also told The Associated Press that he saw workers repairing a pipeline. He spoke on condition of anonymity because of the sensitivity of the situation.

With over 260 billion barrels of proven oil reserves, a quarter of the world's total, Saudi Arabia is OPEC's largest producer and the top foreign supplier to the United States.

The kingdom maintains crude oil production capacity of up to about 11 million barrels a day. The Abqaiq facility processes up to about 7 million barrels a day, 93 percent of which is loaded onto tankers for export.

Militants have previously attacked oil offices but the attack on Friday, the Muslim day of prayer, marked the first time a processing facility or refinery has come under fire.

On May 1, 2004, attackers stormed the offices of a Houston-based oil company in the western Saudi oil hub of Yanbu, killing six Westerners and a Saudi before security forces killed the attackers. Several weeks later, Al Qaeda-linked gunmen stormed oil company compounds in Khobar, on the eastern coast, and took hostages. Twenty-two people, 19 of them foreigners, were killed before the siege ended.

In December 2004, Al Qaeda leader Usama bin Laden for the first time called on militants to attack oil targets in the Gulf to stop the flow of oil to the West.

No major attacks followed in the region. Some experts have said that Al Qaeda would do nothing to seriously jeopardize the oil industry on which the kingdom's wealth is based, because the group's long-term goal is to run Saudi Arabia.