IS claims bomb attack in Yemen's Aden
An explosives-rigged car was detonated outside the security headquarters in the central district of Khor Maksar in Aden where the internationally-recognised government of President Abd Rabbo Mansour Hadi is based, a high-ranking security official said.
The attackers freed dozens of detainees from their holding cells, some of whom took up arms to fight alongside the militants.
The gunmen also took an unknown number of people hostage inside the unit on Sunday afternoon, with two policewomen killed execution-style by the attackers, an official said.
Earlier on Sunday, Yemeni security sources had said they suspected al-Qaeda militants were behind the well-coordinated attack.The Islamic State group later claimed the attack on the criminal investigations unit in an online statement released by the IS Aden and Abyan Wilayah, which said clashes were "still ongoing".
End to relative calm
The United States, which considers AQAP the most dangerous branch of al-Qaeda, also regularly conducts drone strikes on southern Yemen.
IS has not claimed a Yemen attack in almost one year. The group claimed responsibility for a December 18, 2016 attack in Aden that killed 48 and wounded 84 soldiers who had lined up to get their salaries.
Yemen's conflict escalated in 2015 when a Saudi-led coalition launched a military intervention aimed at rolling back Houthi gains and restoring Hadi to power.
The fighting has caused a humanitarian catastrophe which the UN says is the world's worst, pushing seven million people to the brink of famine and sparking a cholera outbreak that the World Health Organisation says has killed 2,000 people.
More than 10,000 civilians have been killed in the conflict and thousands more have been injured.
Yemen is today split in two, with the Houthi-Saleh camp controlling the north and coalition-backed pro-government forces in the south.