ADEN — A bomb ripped through the car of a senior Yemeni security officer Thursday in the southern port city of Aden, killing him instantly in an attack that another security official blamed on Islamist militants with ties to al Qaeda.

The explosive was stuck to the side of the vehicle and went off as Colonel Abdallah al-Mawzai turned the key in the ignition, the official told Reuters, speaking on condition of anonymity.

The killing of Mawzai, the top security officer in the city's sixth district, was the latest in a string of assassinations of security officials in the region, where Al-Qaeda's Yemeni wing and local affiliates have established a presence.

A group calling itself Ansar al-Sharia (Partisans of Islamic Law), which pledged allegiance to Al-Qaeda in the Arabian Peninsula, seized parts of southern Abyan province last year as an uprising against Yemen's former president, Ali Abdullah Saleh, gained momentum.

The uprising split Yemen's military and raised fears in the United States and Saudi Arabia — both targets of abortive attacks al Qaeda planned from Yemen — that political chaos would embolden Islamists in Yemen.

The US and its Gulf allies brokered a power transfer that replaced Saleh with his deputy, Abed Rabbo Mansour Hadi. He took office in February vowing to fight Al-Qaeda, and launched a US-backed offensive on Islamists in the south in May.

That campaign drove militants from towns they controlled in Abyan, but subsequent assassinations and suicide attacks in Aden and Sanaa showed they have not been crushed.

Separately, a security official in the southern province of Lahj, which is next to Aden, said security forces had captured a number of Ansar al-Sharia fighters leaving Abyan over the past few days.