Suspected Al-Qaeda militants staged twin suicide bombings and took hostages on Sunday in Aden, officials said, striking at the heart of the Yemeni government after suffering a string of setbacks.

Six security officers were killed in the attacks on the security headquarters and criminal investigations unit in the central district of Khor Maksar in Aden, where the internationally recognised government is based.

A high-ranking official with the state security forces said an explosives-rigged car driven by an Al-Qaeda operative blew up outside the security headquarters, killing the six officers along with the driver.

A second attacker was killed in a shootout between police and gunmen that followed.

Militants also stormed the building and set alight files and archives, the official said.

Moments later, gunmen stormed the Aden criminal investigations unit, next door to security headquarters, as a suicide bomber detonated an explosive belt in the building, a source in the unit said.

Another attacker was also killed at the scene.

The gunmen took hostage an unknown number of police detectives in the criminal investigations unit, the security source said.

Two policewomen were killed in execution-style slayings by the attackers, who were also believed to be linked to Al-Qaeda.

Sunday's attacks spell an abrupt end to a period of relative calm that has reigned in Aden, where the government of President Abedrabbo Mansour Hadi has been based since it was driven out of the capital Sanaa by a rival rebel camp in 2014.

Yemen's complex war, which pits the Saudi-backed Hadi government against former president Ali Abdullah Saleh and his Houthi rebel allies, has allowed Al-Qaeda in the Arabian Peninsula to flourish in the south of the country.

Yemeni forces allied with a Saudi-led coalition have closed in on AQAP strongholds in recent weeks, driving them out of pockets of the southern provinces of Abyan and Shabwa.

Special forces trained by the United Arab Emirates and backed by the United States launched a “major operation” in early August against the militants in Shabwa province.

A senior military official said at the time that the militants had staged a “tactical retreat” from the province without engaging in any major clashes.

The military operations have also seen the Al-Qaeda insurgents pull back from parts of Abyan province.The United States, which considers AQAP the most dangerous branch of Al-Qaeda, also regularly conducts drone strikes on southern Yemen.

More than 8,600 people have been killed in Yemen since Saudi Arabia and its allies joined the war in 2015 to support government forces, according to the World Health Organization.