Al-Qaeda has claimed an attack on a Yemeni army headquarters in a tightly secured district of Aden in which 20 people died, most of them militants, a monitoring group said Thursday.

The building targeted in Wednesday's attack is located in the heavily patrolled coastal district of Tawahi that hosts intelligence and political police headquarters, a naval base and a presidential residence.

The Al-Qaeda in the Arabian Peninsula (AQAP) affiliate of the global jihadist network claimed the attack in an online message, said the US-based SITE Intelligence Group, which monitors extremist organisations.

AQAP, which the United States views as Al-Qaeda's most dangerous branch, said the attack killed "nearly 50 soldiers" and was part of its campaign "to "target the joint operation rooms that manage the US drones in the country".

Washington has stepped up drone strikes in Yemen against the group in recent months.

Militants launched Wednesday's attack on the northern side of the army headquarters, with some climbing a wall into the building as a suicide car bomber rammed his vehicle into a western entrance, Yemen army officials said.

Reinforcements from the 31st Armoured Brigade stationed in Aden were dispatched to support the troops in Tawahi, according to one official, who added that the fighting went on for several hours.

Six soldiers were killed and 14 wounded, while three civilians including a seven-year-old child also died, the official added. Ten of the assailants plus the suicide bomber died.

Despite repeated attempts by the army to crush AQAP in its strongholds in the east and south, militants continue to carry out deadly attacks on security forces across the country.

The brazen attack on such a highly protected area came despite the authorities having stepped up measures in recent weeks to contain a deadly wave of violence rocking the Arabian Peninsula country for years.

Wednesday's assault is similar to one carried out by gunmen from Al-Qaeda-affiliated group Ansar al-Sharia on an army headquarters in Hadramawt in September, in which they took hostages and 12 people died.

In December, Al-Qaeda militants launched a daylight assault on the defence ministry, killing 56 people.

The group has taken advantage of the weakening of the central government since 2011, as a result of a popular uprising that toppled president Ali Abdullah Saleh after 33 years in power.