Security officials say gunmen on a motorcycle shot dead the head of security in Yemen's port city of Aden.

The officials say Col. Abdelhakim al-Sanidi was killed Sunday while driving on a busy street in al-Mansoura neighborhood.

Last week, high-ranking police officials in Aden said hundreds of al-Qaida militants and Islamic extremists moved into the port city, exploiting a vacuum left by pro-government forces pushing north to fight Shiite rebels and their allies.

No group immediately claimed Sunday's attack, but some officials say they suspect al-Qaida because hitting a moving target from a speeding motorcycle requires specialized training, and because the group has used motorcycles in previous assassinations.

All officials spoke on condition of anonymity because they are not authorized to brief reporters.