At least two cars carrying explosives were fired on at the plant, Saudi officials have said. BBC security correspondent Frank Gardner says the attack is the first direct assault on Saudi oil production.

The al-Qaeda network on the Arabian Peninsula has long called for attacks on Saudi oil installations.

Saudi Oil Minister Ali al-Nuaimi said output at the facility, which handles about two-thirds of the country's oil production, was unaffected by the attack.

Oil security analysts have estimated that a serious attack on the facility could halve Saudi exports for up to a year.

On news of the attack, the price of crude oil for April delivery leapt as much as 3.4% to $62.60 a barrel on the New York Mercantile Exchange, its biggest gain since 17 January.

Former Middle East CIA field officer Robert Baer has described Abqaiq as "the most vulnerable point and most spectacular target in the Saudi oil system."

Abqaiq handles crude pumped from the giant Ghawar field and ships it off to terminals Ras Tanura -- the world's biggest offshore oil loading facility -- and Juaymah.

"It's not clear what damage there is but Abqaiq is the world's most important oil facility," said Gary Ross, CEO at PIRA Energy consultancy in New York.