SANA, Yemen — Suspected Al Qaeda militants attacked a Yemeni intelligence headquarters on Saturday, killing 20 people in a bold attack in the nation’s main southern city of Aden, officials said.

The attack in the heart of the port city underscored Al Qaeda’s ability to launch deadly strikes despite a two-month Yemeni military offensive supported by the United States that earlier this year dislodged militants who had taken control of a string of southern towns near Aden.

Militants attacked the intelligence building from two sides, firing automatic weapons and rocket-propelled grenades, according to intelligence officials in the city and witnesses from the adjacent state TV and radio building. By the end of the day, 20 were dead. All were military and security men except for one civilian.

Al Qaeda-linked militants took advantage of political turmoil in Yemen to overrun several major towns in Abyan province, neighboring Aden. They held many of them for months until the military drove them out of most areas in May, including the Abyan provincial capital of Zinjibar and the nearby town of Jaar. More than 100,000 people fled the violence there, with many taking refuge in makeshift shelters and schools in Aden.

Many of the militants escaped into nearby mountains, however, and have continued to carry out attacks. Suicide bombings and assassinations have targeted officials in Aden tasked with fighting Al Qaeda. An Al Qaeda front group, Ansar al-Shariah, was behind the kidnapping of a Saudi diplomat in the port of Aden in March.

Militants are still able to mount assaults despite a military offensive.

The area has had other violence as well. Earlier this week, gunmen stormed a passenger plane after it landed in Aden and grabbed  an opposition leader from his seat and spirited him away to an unknown destination.

The masked gunmen burst into the airport building first, meeting no resistance from airport security. They then ran onto the runway and boarded the plane to kidnap retired Major General Ahmed Abdullah al-Hassani, a former Yemeni navy commander and a prominent campaigner for the south’s secession.

It is not clear who was behind the abduction.

The United States considers Al Qaeda in the Arabian Peninsula as the terror network’s most dangerous offshoot, held responsible for several failed attacks on US territory.