ADEN - Saudi-led forces attacked al Qaeda militants in Aden overnight in an effort to dislodge them from a stronghold in the southern Yemeni port city, killing at least 17 people in helicopter strikes, medics and a security official said on Sunday.

On another front, the Saudi-backed forces supporting President Abd-Rabbu Mansour Hadi fought to widen their control of Taiz in the southwest after breaking a seige there on Friday.

Islamist militants from al Qaeda and Islamic State have exploited the conflict to expand their control, especially in areas where Hadi supporters have managed to expel the Iran-allied Houthis, including in Aden and Lahej provinces.

Witnesses and medics said helicopters from the Saudi-led coalition struck armoured vehicles and a government compound used by the militants in al-Mansoura district, a stronghold in north Aden. There was no immediate comment from the coalition.

Medics said at least 17 bodies were taken to hospitals. The sources said at least 20 civilians and militants were also wounded as well as three members of the security forces.

There was tenuous calm in the district later on Sunday, with streets around the area closed off. A Reuters reporter saw the rubble of a butcher's shop and the wreckage of cars. Electricity was cut across the district and food shops remained closed.

Shops and businesses closed as security forces sealed off a block in the area, where dozens of suspected militants are believed to be holed up. The city's governor said the operation was the second phase of a government campaign.

"This stage will continue until it achieves its objectives, foremost of which to impose the authority of the state and restore security and stability in all districts of the capital Aden and its suburbs and to end the security chaos," Governor Aydaroos al-Zybaidi said.

Militants killed Zybaidi's predecessor and several other government officials, military and security officers in a series of suicide attacks and shootings in Aden since Hadi's forces captured the city from the Houthis last July.

The campaign took place a day after forces loyal to Hadi broke a siege by the Houthis around Taiz, Yemen's third biggest city, about 200 km (124 miles) northwest of Aden.

The advance represented a breakthrough for Hadi's government, which has been struggling to achieve a major victory against the Houthis who seized control of most of the country in 2015 in what they described as a revolution against corruption.

Residents said that more fighting was reported in eastern Taiz on Sunday between the Houthis and fighters allied to Hadi.

Witnesses said there were many casualties but gave no figures or more details.

Yemen's government was forced out of the capital Sanaa by the Houthi rebels in September 2014 and is now based in Aden, but struggles to impose its authority even there. The coalition began a military campaign in March last year to prevent the Houthis from taking complete control of Yemen.