Six alleged al Qaeda members, including a key suspect being hunted by Yemen on suspicion of links to Osama bin Laden , were killed when their car exploded, a source close to Yemen's government said on Monday. The source said it was not yet clear what caused Sunday's car explosion in the Marib oil-producing province, 170 km (105 miles) east of the capital Sanaa. But he said the car appeared to have been carrying explosives.

"All six were killed in the explosion. The cause is not yet known," the source told Reuters.

There has been no official comment so far on the blast and the source said security forces were still investigating.

He said the six men in the car were Yemenis suspected of links to bin Laden's al Qaeda, blamed by Washington for last year's September 11 attacks on the United States.

The source said one of the men was Qaed Senyan al-Harthi, also known as Abu Ali, one of two key suspects that local security forces have been trying to hunt down for questioning since the hijacked plane attacks on New York and Washington.

Abu Ali is suspected of involvement in the 2000 suicide bombing of the U.S. destroyer Cole in Yemen's Aden harbor that killed 17 U.S. sailors.

Yemen is keen to shake off its reputation as a haven for Muslim militants and says it is holding 85 people arrested in a manhunt for al Qaeda members.

The rugged mountainous area between Sanaa and Marib is a stronghold for armed tribesmen blamed by the government for many kidnappings of tourists and foreigners working in Yemen in recent years.

The car blast coincided with a visit to Marib by the U.S. ambassador to Yemen who returned to Sanaa on Monday as scheduled, a source close to the U.S. embassy said.

He said there was no link between the visit and the explosion or a shooting on Sunday involving a helicopter owned by U.S. firm Hunt Oil operating in Yemen.

Gunmen fired at the helicopter shortly after it started a journey from Sanaa to Marib, but it landed safely with only one person slightly hurt.

Officials have said it was not yet known who had fired at the helicopter and that authorities were questioning some tribal leaders in the area near the airport in the capital.

A French supertanker was holed by an apparent attack off the coast of Aden last month, almost exactly two years after the Cole attack, which Washington has also blamed on bin Laden.

Yemen is the ancestral home of the Saudi-born fugitive.

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Six alleged al Qaeda members, including a key suspect being hunted by Yemen on suspicion of links to Osama bin Laden, were killed when their car exploded, a source close to Yemen's government said on Monday.