The Interior Ministry said four bombers were killed when security forces blew up four rigged cars at dawn before they reached their targets. A guard was also killed.

There was no damage to the state-owned facilities, the ministry added. Yemen, a minor oil producer, is the ancestral home of al-Qaeda leader Osama bin Laden and has been battling militants for years.

"The Interior Ministry foiled early this morning two terrorist attacks involving four cars that targeted an oil exporting terminal in Hadramout and an oil refinery and gas production unit in Marib," a statement carried on state media said.

The attempted attacks occurred east of the capital Sanaa.

There was no immediate claim of responsibility. Yemen said it had launched an investigation.

Militants from al-Qaeda and other groups regard Western allied governments such as Yemen's as traitors and therefore legitimate targets.

The statement said that in the first attack, one of the bombers was disguised as a military officer while the second attacker was wearing the uniform of workers at the terminal.

It said a small fire broke out at one of the storage tanks in Hadramout, and it was quickly brought under control.

In the Marib attack, two cars tried to storm the refinery and gas production unit, the ministry said.

"The suicide terrorist attack did not result in any loss of life or damage to the refinery or gas production unit in Marib, whereas both suicide attackers were killed on site," it said.

Yemen, which produces around 400,000 barrels per day of crude oil, is due to hold presidential and municipal elections on September 20.

The impoverished country has vowed to crack down on attacks by al-Qaeda-linked militants and kidnappings of foreigners by disgruntled tribesmen.

The Arab country joined the US-led war on terrorism after the September 11 attacks on the United States in 2001.

On Monday, al-Qaeda's deputy leader Ayman al-Zawahri called on Muslims to strike Western interests and stop what he called the theft of Muslim oil by Western countries.

"The strongest way to aid our Muslim brothers ... is to strike the interests of Jews and Crusaders and those who cooperate with them," Zawahri said in a video posted on a Web site used by Islamist militants.

"There must be a focus on their economic interests and in particular on stopping the theft of Muslims' plundered petroleum," he said.

Militant attacks off the Yemeni coast have included the bombing in 2000 of the US warship Cole and an attack in 2002 on the French supertanker Limburg. In neighbouring Saudi Arabia, which has also been battling al-Qaeda, suicide attackers tried to storm the world's biggest oil processing plant in Abqaiq in February.

Yemen, on the southern corner of the Arabian Peninsula, has been widely seen in the West as a haven for Muslim militants, including al-Qaeda supporters.

Western diplomats say some of the militants are war veterans who fought Soviet forces in Afghanistan in the 1980s. Many are protected by powerful tribal leaders in mountain regions outside the central government's control.