The official, a high-ranking member of the security forces, said an explosives-rigged car driven by an Al-Qaeda operative blew up outside the security headquarters in the central district of Khor Maksar in Aden, where the internationally recognised government is based.
Moments later, gunmen stormed the Aden criminal investigations unit and set alight files and archives, as a suicide bomber detonated an explosive belt in the building, a source in the unit said.
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 being 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 Iran-backed Huthi 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.
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.
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