A booby-trapped car drove into an army checkpoint outside of Yemen's southern port city of Aden and exploded in an apparent suicide attack that killed at least six people and injured 15 on Sunday, police and medical sources said.

Aden lies east of a shipping strait, where some 3 million barrels of oil pass daily.

"The initial evidence we've gathered suggests it was a suicide bomb attack," a local police source told Reuters.

The blast comes days after a car rigged with explosives blew up and killed a British ship surveyor in Aden, which officials said was a targeted attack against the long-time resident.

Witnesses to the checkpoint attack on Sunday said they saw a car speed into a street cordoned off by armoured vehicles, setting at least two armoured vehicles ablaze.

"The car crashed into a military armoured vehicle, which exploded and caught fire. The soldiers started firing heavily," a witness said.

Security forces sealed off the al-Harba area of Aden and witnesses said they could still hear heavy gunfire.

Aden sits to the west of the flashpoint province of Abyan, which has descended into daily bloodshed since militants seized at least two cities and a makeshift military base, forcing some 54,000 residents to flee to Aden for refuge.

One security source said the vehicles targeted in the Sunday attack were part of a convoy of forces being sent to join the military operation in Abyan, which has lasted for more than a week.

The unrest in the south has erupted during the mass protests seeking to end President Ali Abdullah Saleh's 33-year rule. Saleh is convalescing in Riyadh after a bomb blast in his presidential compound in June.

Reuters