At least 57 civilians were killed on Sunday when Shiite rebels bombarded Yemen's second city Aden, where Saudi-backed pro-government forces have made gains against the insurgents, a health official said.

The bloodshed came two days after Prime Minister Khaled Bahah declared the city to be "liberated", although Iran-backed rebels continued to resist in some districts.

Local health chief Al-Khader Laswar told AFP that the death toll had risen to 57 from 43, and that 12 children and six women were among the dead.

More than 215 people, also including women and children, were wounded when the Dar Saad neighbourhood in the north of the port city was targeted, Laswar said.

Backed by air support from Saudi-led warplanes and troops freshly trained in the kingdom, forces loyal to exiled President Abedrabbo Mansour Hadi have managed to wrest back control of most of Aden.

Two ministers from the government exiled in Saudi Arabia returned to Aden this weekend, and on Sunday they toured the southern city devastated by four months of ferocious fighting.

On Sunday, fighters from the pro-Hadi Popular Resistance advanced towards the rebel-held district of Al-Tawahi, a military source said.

- Air strikes hit rebels -

Aden's presidential residence is in the district, where Hadi took refuge after escaping house arrest under the rebels in Sanaa in February, before then being forced to flee for Saudi Arabia.

Warplanes from a Saudi-led Arab coalition have pressed an air campaign launched in March in support of Hadi and against the Huthis and renegade troops loyal to former president Ali Abdullah Saleh.

Since late Saturday, around 15 air strikes targeted rebel positions in Al-Tawahi and on the northern outskirts of the city where the rebels had brought in reinforcements, military sources and witnesses said.

There was also fighting in the Crater district where some rebels remain holed up, according to pro-Hadi fighters.

Nine rebels were killed in a raid on Khormaksar neighbourhood, witnesses said.

The interior and transportation ministers headed a delegation from the exiled government that landed in Aden on Friday night.

On Sunday they also took part in a meeting that looked into reopening the airport and the port to allow the flow of much-needed aid, as well as the restoration of electricity and water services, state media said.

Loyalist forces took the airport shortly after an assault dubbed "Operation Golden Arrow" began on Tuesday.

- 'There is no life!' -

Some displaced residents have returned to assess the damage to their houses and neighbourhoods.

"There is no life! No hospitals, no electricity, nor water. If it was not for the two wells of the neighbourhood, people would have died of thirst," said Crater resident Moatez al-Mayssuri.

A rebel spokesman dismissed the government's claims on Saturday that it now controlled Aden as "psychological warfare and an attempt to improve the crushed morale" of loyalist fighters.

On Sunday, the spokesman for the Huthis' Ansarullah movement Mohammed Abdulsalam said the rebels had "regained the lead and repelled several attacks by the mercenaries".

The rebels, meanwhile, also targeted Saudi positions across Yemen's northern border in Najran and Jizan, according to the rebel-controlled Saba news agency which cited a military source.

Elsewhere, firefighters managed to extinguish a huge blaze at a gas depot southwest of Taez in central Yemen after it was shelled by rebels, according to the government-run news agency.

The United Nations has declared Yemen a level-3 humanitarian emergency, the highest on its scale.

After weeks of shuttle diplomacy between the two sides, the UN announced a humanitarian truce last weekend to allow the delivery of desperately needed relief supplies, but the ceasefire failed to take hold.

More than 21.1 million people -- over 80 percent of Yemen's population -- need aid, with 13 million facing food shortages.

Upwards of 3,200 people have been killed in the fighting -- many of them civilians, the UN says.