Saudi-led helicopters attacked al Qaeda militants in Aden overnight in an effort to dislodge them from a stronghold in the southern port city, killing at least 18 people, medics and a security official said on Sunday.

The assault took place as Saudi-backed forces supporting President Abd-Rabbu Mansour Hadi fought to widen their control of Taiz in the southwest of Yemen after breaking a siege 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 Apache 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 a total of 18 people have been killed -- 17 suspected militants and one civilian bystander -- and at least 23 civilians and militants were wounded. Three members of the security forces were also injured.

Security forces cordoned off an area of al-Mansoura district where dozens of suspected militants are believed to be holed up, while warplanes from the Saudi-led coalition dropped leaflets on the area telling residents to stay home and report any militants to authorities.

A tenuous calm in the district was broken occasionally in the afternoon with bursts of gunfire. 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 to restore state control over the city, the temporary seat of the Yemeni government.

"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-Zubaidi said.

Militants killed Zubaidi'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.

More fighting in Taiz

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, where reinforcements from both sides have arrived.

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

Medical supplies provided by Saudi Arabia have also reached the main al-Thawra hospital in Taiz for the first time since last year, a medic at the facility said.

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.