In a sign of a major Houthi retreat from the southern capital Aden, several Yemeni ministers returned to the city on Thursday from exile in Saudi Arabia, three months after being pushed out by the Shia militia's advance.

The returnees included the ministers of the interior and transport, the former interior minister, the intelligence chief, and the deputy head of the house of representatives, Reuters reported.

"(Exiled President) Abd Rabbuh Mansour Hadi delegated this group to return to Aden to work to prepare the security situation and ensure stability ahead of a revival of the institutions of state in Aden," a Yemeni official told Reuters.

They arrived by helicopter at a military air base in Aden, after loyalists of Yemen's exiled president recaptured the city's airport and seized more ground on Wednesday as they pressed their biggest fightback yet against the Houthi militia.

The offensive, dubbed Operation Golden Arrow, is the first major advance by the loyalists since the Iranian-backed Houthi militiamen entered the port city in March, forcing President Hadi and his government into exile in neighbouring Saudi Arabia.

Yemeni government spokesman Rajih Badi told AFP late on Wednesday that the ministers of interior, health, and transport "will return in the coming hours" to Aden "to assess the situation and the magnitude of the destruction".

"Most parts of the city are secure," he said, adding that the government is working on securing the rest of Aden, Yemen's main southern city.

Despite an appeal from US President Barack Obama to Saudi King Salman for an urgent end to the fighting, Saudi-led warplanes carried out six raids on militia positions before dawn, witnesses and military sources said.

Popular Resistance fighters - southern forces that have been the mainstay of support for Hadi - recaptured the provincial government headquarters in the Mualla district opposite Aden's main commercial port, their spokesman Ali al-Ahmadi told AFP.

They also advanced in Aden's Crater downtown district, where a presidential palace is located, amid heavy fighting, he added.

And pro-government fighters entered the small commercial port in Mualla itself, near the main port which the Houthi militiamen had failed to take, according to military sources.

General Fadhl al-Hasan, leading pro-government forces' operations in Aden's west, told AFP that his troops have also captured the coastal road overlooking the strategic Bab el-Mandeb Strait between Yemen and Djibouti.

The road links Aden to the rebel-held city of Mocha, home to a port near Bab el-Mandeb.

"The Resistance is at the gates of the city of Mocha," through which Houthi militia reinforcements used to arrive from northern cities under their control, Hasan said.

Badi said that "the majority of the security leaderships are in Aden and they are leading the battles," adding that others will "soon return".

He also urged the Red Cross "to take the Houthis who have surrendered" to the pro-government forces.

Houthi 'collapse'

On Tuesday, the Popular Resistance, backed by reinforcements freshly trained and equipped in Saudi Arabia, retook the airport and much of the surrounding Khormaksar diplomatic district.

"After the recapture of Khormaksar, there was a collapse in the ranks of the Houthis and their allies," renegade troops loyal to Hadi's predecessor Ali Abdullah Saleh, Ahmadi said.

It was the defection of the 39th Armoured Brigade on 25 March that had enabled the Houthi militia to take the airport.

A Western diplomat in Riyadh told AFP that, if the airport and environs can be secured, it could be used to deliver supplies to loyalist forces.

"This could be the first step to a beachhead," he said.

Much of Aden has been reduced to rubble by four months of ferocious fighting.

The retreating Houthis pounded residential districts in the north and east of Aden with Katyusha multiple rocket launchers, provincial officials said.

At least 12 civilians were killed and 105 wounded, Aden health department chief Al-Khader Laswar told AFP.

Eight loyalist fighters were killed and 30 wounded in the fighting, Laswar added.

There was no immediate word on Houthi militia losses.

The offensive comes after the failure of a UN-declared truce that was to have taken effect just before midnight on Friday to allow the delivery of desperately needed relief supplies.

The White House said Obama spoke by telephone with the Saudi king on Tuesday "about the urgency of stopping the fighting in Yemen and the importance of ensuring that assistance can reach Yemenis on all sides of the conflict".

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

More than 21.1 million people - over 80 percent of Yemen's population - need aid, with 13 million facing food shortages, while access to water has become difficult for 9.4 million people.

The conflict has killed more than 3,200 people since late March, according to UN figures.