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Militants attacked the intelligence building from two sides, firing automatic weapons and rocket-propelled grenades. (Reuters)

The death toll of a suspected al-Qaeda attack on a Yemeni intelligence headquarters rose to 20 on Saturday, in the worst such attack in a year that highlights the challenges faced by the country's new leadership as it struggles to bring security and reconcile a military with split loyalties.

The attack, in the heart of the port city of Aden, underscored al-Qaeda's ability to launch deadly strikes despite a two-month Yemeni military offensive backed by the U.S. that earlier this year dislodged militants who had taken over a string of southern towns near Aden.

In a co-ordinated attack, two groups of masked militants stormed 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.

While one group clashed with guards of the intelligence building's main gate, a second threw a bomb at a small mosque, killing soldiers who were resting and sleeping inside, officials said. The gunmen then sprayed their victims with bullets before detonating a car bomb in front of the intelligence building, collapsing its facade.

Witnesses said they saw gunmen open fire on three soldiers at a front gate, killing them on the spot, before launching rocket-propelled grenades at the building and mistakenly hitting the television offices. Two female reporters were critically wounded, witnesses said.

By the end of the day, 20 were dead. All were military and security men except for one civilian, while six other civilians were injured aside from the reporters.

The same intelligence building had come under attack in 2010 by al-Qaeda. Saturday's attack, which took nearly 45 minutes, carries the fingerprints of the group, a security official said.

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 U.S. territory.

Al-Qaeda-linked militants took advantage of political turmoil in Yemen to overrun several major towns in Abyan province, neighbouring Aden. They held many of them months until the military drove them out of most areas since the offensive started 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 top officials in Aden tasked with fighting al-Qaida. An al-Qaida front group, Ansar al-Shariah, was behind the kidnapping of a Saudi Arabian diplomat in the port of Aden in March.

The area sees 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 Maj. Gen. 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 U.S. territory.