The Islamic State group claimed a major attack on Yemen's government bastion of Aden on Sunday which killed at least 15 people, wounded 18 others and sparked a hostage crisis.

IS and its extremist rival Al-Qaeda have taken advantage of the war between Yemen's Saudi-backed government and the country's Shiite Huthi rebels, who are allied with Iran, to bolster their presence across much of the south.

While Al-Qaeda in the Arabian Peninsula (AQAP) has for years been the stronger presence in southern Yemen, IS has recently come forward to claim attacks on both the army and the country's Shiites, whom it considers heretics.

IS claimed Sunday's attack on the criminal investigations unit in an online statement released by its "Aden and Abyan province", which said clashes were "still ongoing".

Security officials in the southern province of Aden, where the Yemeni government is based, said an explosives-rigged car blew up outside the investigations unit, killing six officers on the spot along with the driver.

Around 30 gunmen then stormed the unit and freed around 50 detainees from their holding cells, some of whom took up arms to fight alongside the militants, according to a high-ranking official.

The security officials said a total of four suicide bombers blew themselves up during the attack, one of them targeting the convoy of Yemen's chief of security.

The gunmen also took an unknown number of people hostage inside the unit on Sunday afternoon. Two policewomen were killed execution-style by the attackers, the official said.

By late afternoon, security officials said four policemen -- among them a colonel -- had been freed.

Earlier in the day, Yemeni security sources had said they suspected Al-Qaeda militants were behind the well-coordinated attack.

- End of the calm? -

The assault spells 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 rebel-held capital Sanaa in 2014.

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

IS has not claimed a Yemen attack in almost a 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.

While southern Yemen has long been the target of a drone war by the United States, which classifies AQAP as the radical network's most dangerous branch, Washington appears to have recently turned its aim towards IS as well.

In October, a US drone strike targeted what local officials said was an IS group in the central Bayda province -- the second known strike by Washington against the jihadists in Yemen.

The United States has ties to Yemeni special forces trained by the United Arab Emirates, a key member of the Saudi-led coalition, who have closed in on AQAP southern strongholds in recent weeks.

The UAE has played a key role in the Saudi-led military campaign to prop up Hadi's government against both AQAP and the Huthis, who forced him into exile back in 2014.

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.

Another 2,100 have died of cholera this year.

Long the poorest country in the Arab world, Yemen now stands steps away from an official declaration of famine, as war, displacement, and port and airport blockades leave its people struggling to secure food.

All parties in the Yemen war have drawn harsh criticism for their failure to protect civilians, but the Saudi-led coalition allied with the government has been singled out by the United Nations over civilian casualties.

In September, the UN Human Rights Council announced an agreement to send war crimes investigators to Yemen.

The UN humanitarian coordinator in Yemen, Jamie McGoldrick, in a statement on Sunday said he was "horrified" by the continuing violence.

source: AFP