Coalition officials said on Friday that they were responding to intelligence that suggested that Al-Qaeda units might target the kingdom's Ras Tanura terminal, the world's biggest offshore oil export facility, and Bahrain's Bapco refinery.

"Acting on information received, Coalition naval forces, operating in support of Saudi and Bahraini forces have deployed units to counter a possible maritime threat to the oil facilities at Ras Tanura," a spokesman for Britain's royal navy units based in Dubai said in a statement on Friday.

Saudi navy on guard

Saudi Arabia's own security forces and navy are also guarding the strategic oil facilities.

Coalition forces are patrolling only in international waters.

"Coalition forces are taking the prudent, precautionary measures and focusing on maritime security operations in the Gulf on these possible threats," Kevin Aandahl, a spokesman for the US naval forces central command in Bahrain said.

Ayman Al-Zawahiri has told his followers to attack oil facilities 

"We're constantly and routinely conducting maritime security operations in the Gulf and international waters and these operations deny terrorists the use of the maritime environment as a venue for attack."

Saudi officials said that Al-Qaeda had a proven record of attacking oil facilities.

"Ras Tannura or any other oil installation is a high-probability  potential target for terrorist threats, but I cannot confirm or deny a specific new threat" of attack against an oil facility, General Mansur al-Turki, the spokesman for the Saudi interior ministry, said.

Oil prices jump on news

Oil prices initially rose about 30 cents to around $61 on the news.

Crude oil shipments were continuing as normal from the kingdom's main east coast terminal, industry sources said.

Riyadh is exporting around 7 million barrels per day (bpd) of oil to world markets, with Ras Tanura handling most of it. The terminal has the capacity to export 6 million bpd.

In February Al-Qaeda launched a unsuccessful attack on Saudi Arabia's huge Abqaiq facility, the world's biggest oil processing plant.