Pro-government forces battled retreating rebels on the northern outskirts of Yemen's second city of Aden on Sunday ahead of a humanitarian truce declared by the Saudi-led coalition bombing the Iran-backed insurgents.

Troops loyal to exiled President Abedrabbo Mansour Hadi sought to tighten their control of the southern port city and surrounding areas ahead of the ceasefire, scheduled to take effect at midnight (2100 GMT).

But Houthi rebel leader Abdulmalik al-Houthi rejected the unilaterally declared truce, according to a statement on a Twitter account believed to be managed by his group, saying it was aimed at allowing pro-government fighters to regroup.

It was not immediately possible to contact the Houthis to confirm the remarks.

Impoverished Yemen has been rocked by months of fighting between Houthi Shiite rebels and Hadi loyalists, supported by the Saudi-led Arab coalition, leaving thousands dead and many more needing urgent aid.

Pro-Hadi Popular Resistance militiamen attacked the Houthis overnight Saturday on the northern outskirts of Aden, forcing the rebels out of the Basateen and Jawala areas.

The loyalist forces have been bolstered by new weaponry and armoured vehicles delivered by the coalition.

They also benefited from coalition air support, military sources said, adding that dozens of rebels were killed in the latest fighting.

Seven pro-Hadi fighters were also killed and 29 were wounded, a medical source said.

Further north, troops loyal to Hadi forced rebels out of the town of Sabr in Lahj province, General Fadhel Hassan told AFP.

Hassan said pro-Hadi troops had taken the town that links Aden to Huta, the provincial capital of Lahj, adding that Huta is the next target before reaching Al-Anad, the country's largest airbase.

The strategically important base housed US troops involved in a long-running drone war against Al-Qaeda before the fighting forced them to withdraw.

Coalition air strikes killed 17 rebels in Lahj on Sunday and 14 in Abyan province, loyalist military sources said.

In a sudden turn of events, pro-Hadi forces last week regained control of much of Aden, which Houthi rebels overran in March.

Troops trained and armed by the coalition appeared to have triggered the shift in the balance in the Hadi loyalists' favour.

The Houthis and allied renegade forces loyal to former president Ali Abdullah Saleh advanced on Aden after Hadi took refuge in the city following his escape from house arrest in Sanaa in February.

He later fled to Saudi Arabia, which assembled an Arab coalition that began an air campaign in late March against the rebels in a bid to restore the UN-backed leader.

In Riyadh, Hadi on Sunday received the UN envoy to Yemen, Ismail Ould Cheikh Ahmed.

The two "discussed coordination over humanitarian aid delivery within the framework of the declared truce", a Yemeni presidency source said.

The Saudi-led coalition announced the five-day truce to allow aid deliveries, but also said it reserved the right to respond to "military activity or movement".

The truce was announced at Hadi's request, it said.

A UN-declared six-day truce failed to take hold earlier this month after it was ignored by both the coalition and the rebels.

The coalition said at the time it did not receive a request to halt operations from Hadi. UN chief Ban Ki-moon said he was "disappointed" the truce failed.

But desperately needed relief supplies have recently begun to trickle into Aden after pro-Hadi fighters secured the city.

A ship carrying 3,000 tonnes of supplies from the UN's World Food Programme docked in Aden on Tuesday, the first UN vessel to reach the city in four months of fighting.

Other ships from the UN and Gulf countries followed.

On Saturday, a WFP ship carrying 3,400 tonnes of mixed food supplies arrived in Aden, WFP spokeswoman Reem Nada told AFP, adding that the shipment was enough to feed 192,000 people for a month.

On Sunday, a Saudi vessel loaded with 4,000 tonnes of food aid docked at Aden port at midday, an AFP correspondent said.

The United Nations says the conflict has killed more than 3,640 people, around half of them civilians, since late March.

On Friday, the International Committee of the Red Cross warned that civilian suffering in Yemen had reached "unprecedented levels".