The bombers aimed their attacks within half-an-hour of each, at an oil refinery in Maarib, a lawless tribal area in the desert east of Sanaa, the capital, and a Canadian-run terminal in the south-eastern port of Dhabba, on the Gulf of Aden.

They were killed when security forces blew up the four cars at dawn before they were able to reach their targets, said the Interior Ministry. A security guard was also killed.

There was no immediate claim of responsibility from any group.

The attempted attacks, came just five days before presidential elections in which President Ali Abdullah Saleh faces his first serious challenge since he became head of state in 1978. They also followed a videotaped message this week from Ayman al-Zawahiri, al-Qaeda’s No. 2, warning of forthcoming attacks on the Gulf and on facilities he blamed for stealing Muslim oil.

"The two attempts, at around 5:30 am local time (03.30 BST), were foiled by the forces in charge of security at oil installations and the Yemeni army," a security source said.

In the first attack, two suicide bombers drove "at great speed" toward the Dhabba Port at 5:15 a.m. (0415 BST) in an attempt to blow up oil storage tanks, a statement from the Interior Ministry said. The port has previously been the scene of an attack in 2002 on the French tanker Limburg.

The driver of the first car was dressed in a uniform similar to those worn by staff at the facility, and the second driver was in a military uniform. Guards at the port were able to blow up the rigged cars before they reached their targets, and a security guard was also killed

In the second attack, which authorities said took place at 5:50 a.m. (0450 BST), security guards blew up two white cars loaded with explosives. The vehicles were similar to those driven by staff at the facility, according to the Ministry statement. The two bombers were killed and no one else was injured.

Officials said that an investigation was under way to determine the identity of the bombers. Fourteen of 23 al-Qaeda prisoners who escaped from jail in February are still at large.

The run-up to the September 20 presidential and local polls in Yemen, a tribal-based nation and one of the poorest the world, has been blighted by violence.

Unlike its wealthy Gulf neighbours, Yemen is only a modest producer of oil, and is not a member of the Opec oil cartel. The ancestral homeland of Osama bin Laden, it has become known as a haven for terrorists, but since the September 11 attacks authorities have been working with Washington to clamp down on suspected al-Qaeda sympathisers.

In October 2000, 17 US sailors died when suicide bombers aboard a small boat attacked the destroyer USS Cole off the southern port of Aden in an attack claimed by the terror organisation.

An Interior Ministry statement said the violence and threats would not stop Yemen from "continuing its persistent efforts to fight terrorism and the terrorist elements of darkness that are trying to undermine the security and stability of the country and its interests."

On Sunday, tribesmen abducted four French tourists in the south east of the country in an attempt to put pressure on authorities to free jailed members of their tribe.

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