Saudi-led warplanes bombed Yemeni rebels who clashed with pro-government fighters in the south on Monday despite a UN-declared truce aimed at delivering desperately needed aid, military sources said.

UN chief Ban Ki-moon said he was "very much disappointed" by Yemen's failed ceasefire but retained hope that the fighting might still end, his spokesman said.

Elsewhere, 10 civilians were killed in pre-dawn raids on the capital Sanaa, according to medical sources.

The coalition raids targeted positions of the Iran-backed Shiite rebels and their allies, forces loyal to ousted president Ali Abdullah Saleh, in Aden and Lahj provinces.

Air strikes in support of forces loyal to exiled President Abedrabbo Mansour Hadi went on despite a six-day UN humanitarian truce which technically took effect just before midnight on Friday.

The coalition has dismissed the ceasefire, saying it did not receive a request from Hadi's government to halt attacks, while the leader of the Huthi rebels said he did not expect the truce to take hold.

At the United Nations, Ban's spokesman Stephane Dujarric said the world body's chief was "very much disappointed that the humanitarian pause did not take hold over the weekend".

He added that "we continue to reiterate our call for an unconditional humanitarian pause".

"We have not lost hope and discussions are ongoing," Dujarric said.

He also defended the UN's decision to call the ceasefire, saying Ban's Yemen envoy Ismail Ould Cheikh Ahmed "had received the commitments he felt were necessary for us to come out with this announcement".

It was "incumbent on all the parties involved to abide by those commitments", the spokesman added.

In fighting on Monday, the latest coalition air strikes targeted a military engineering building in the Sanaa neighbourhood of Saawan, witnesses said.

Medical sources said the raids killed 10 civilians.

However, the Huthi-controlled Saba news agency gave a death toll of 25, and said around 50 people were wounded in the Saawan raids.

- 'Sophisticated weapons' -

Raids also hit rebel positions on the outskirts of the port city of Aden as well as a convoy in the city's neighbourhood of Khor Maksar, a military source said.

Clashes intensified Sunday in Aden, where rebels have besieged many areas controlled by southern fighters loyal to Hadi and known as the Popular Resistance.

The southern fighters managed to push back the rebels in the coastal area of Ras Amran west of Aden, according to General Fadhel Hasan, a Popular Resistance spokesman.

The fighting left 17 gunmen dead, including 11 rebels, according to Hasan, who said the southern fighters have "received sophisticated weapons from the coalition".

These forces, with coalition air support, on Monday retook a major highway near Al-Waht some 15 kilometres (nine miles) north of Aden after fierce fighting, Hasan told AFP.

He said 40 rebels and 12 pro-Hadi fighters were killed.

Another military source said three air raids struck the rebel-controlled Al-Anad air base in Lahj province north of Aden.

A roadside bomb in Lahj also killed six rebels, an official said.

The United Nations has declared Yemen a level-3 humanitarian emergency, the highest on its scale, with nearly half the country facing a food crisis.

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 UN says the conflict has killed more than 3,200 people, about half of them civilians, since late March.

After the rebels overran Sanaa unopposed in September, they went on to seize control of several regions before advancing on Aden where Hadi had taken refuge after escaping house arrest.