Yemen's government has been battling militants for years

One security guard was also killed in the early-morning attacks on the Safer refinery in Marib and the al-Dhabba terminal in Hadramout.

Two cars were used in both attacks which were foiled by security forces.

There was no immediate claim of responsibility. The attacks come ahead of presidential and local elections.

The attacks also came just a few days after al-Qaeda number-two, Ayman al-Zawahiri, called on Islamic militants to target oil facilities in the Gulf region, the BBC's Heba Saleh says.

Yemen, the ancestral homeland of Osama Bin Laden, has seen attacks in the past claimed by Islamists and has gained a reputation as a haven for militants.

Since the 11 September terror attacks on America, the authorities have cracked down on suspected al-Qaeda members, and the US has supplied equipment and training for Yemen's security forces.

Oil is the chief export of Yemen, one of the poorest countries in the Arab world.

Uniforms

According to the interior ministry, attackers struck at al-Dhabba at 0515 (0315 GMT), aiming to blow up storage tanks filled with a "huge amount" of oil.

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 military dress, it said in a statement.

Guards at the port "managed to blow up the rigged cars before they reached their targets" but a security guard was also killed.

Shrapnel from the exploding cars sparked a small fire in one storage tank but it was quickly put out, the ministry said.

Thirty five minutes later, security guards confronted and blew up two cars loaded with explosives at the Safer refinery, killing two bombers, it added.

The attacks were apparently timed to coincide with shift changes at the installations, security officials were quoted as saying by The Associated Press.