A UN-proposed truce in Yemen, aimed at delivering desperately needed aid to millions threatened with famine, failed to take hold Saturday as clashes and Saudi-led coalition air strikes persisted.

The six-day ceasefire formally went into effect just before midnight Friday, but clashes between Iran-backed Shiite rebels and fighters loyal to exiled President Abedrabbo Mansour Hadi hit several areas, while Saudi-led coalition air strikes targeted rebel positions.

The United Nations hoped the truce would hold until the end of the Muslim fasting month of Ramadan next Friday, allowing sorely needed aid to reach civilians.

But with fierce fighting resuming just hours after the truce was supposed to take effect, attempts to end the violence appeared stillborn as each side accused the other of not taking peace efforts seriously.

The coalition said it "did not yet receive any request from the legitimate Yemeni government asking for a truce or cessation of military operations."

Coalition spokesman Brigadier-General Ahmed al-Assiri said the alliance was "not concerned with this truce because it does not provide a commitment from the Huthi militia".

Coalition warplanes raided Huthi positions in the central city of Taez, where clashes between the rebels and loyalists continued after the truce.

Violence also raged overnight in Taez, with witnesses saying the Huthis had also bombed several districts.

Yemen's official news agency, run by the exiled government, accused the Huthis and allied troops loyal to former president Ali Abdullah Saleh of sending reinforcements to the city ahead of the cessation.

In the south, coalition warplanes also hit rebels in the port city of Aden and nearby Lahj province, witnesses said.

The raids came after the rebels bombed second city Aden, according to Abdullah al-Dayani, a spokesman of southern fighters allied with Hadi.

- 'Final hope' -

Warplanes also struck rebels in the southern Daleh province and in Marib east of Sanaa.

The ceasefire was declared after UN chief Ban Ki-moon received assurances from Hadi and the Huthis that it would be respected.

But rebel chief Abdulmalik al-Huthi said ahead of the deadline that "we do not have much hope for the truce to succeed.

Its success "depends on the commitment of the Saudi regime and is conditioned to a complete end to the aggression."

The spokesman for armed forces allied with the rebels, Brigadier General Sharaf Luqman, welcomed the truce, but said his forces are only "exercising the right of defence."

"Any breach to the truce by the aggressors will push our armed forces to retaliate," he added.

If the truce is respected, the World Health Organization and its partners aim to reach more than 2.37 million people.

"This humanitarian pause will allow WHO and... partners to more effectively scale up their response in affected governorates by reaching populations that have been cut off from care and urgently need life-saving health services," said Yemen representative Ahmed Shadoul.

World Food Programme spokeswoman Abeer Etefa said Friday the truce was "our final hope" to reach the needy.

- 'Extremely hazardous conditions' -

It is now more than a week since the UN declared Yemen a level-3 humanitarian emergency, the highest on its scale, with nearly half the country facing a food crisis.

"It is imperative and urgent that humanitarian aid can reach all vulnerable people of Yemen unimpeded and through an unconditional humanitarian pause," said UN spokesman Stephane Dujarric.

UN aid agencies are ready to scale up operations, although the response to an appeal for $1.6 billion (1.4 billion euros) has been meagre, with just 13 percent of that amount received so far.

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

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

Etefa said the WFP delivered 9,000 tonnes of food to its warehouses in Yemen during the past week, and that the truce was needed to secure its mission.

"We hope to see an effective respect for the ceasefire, and to allow us to reach all parts of Yemen regardless of who controls them," she said.

UNICEF said it was stepping up nutrition screening, vaccinations and other life-saving interventions for millions of children, with teams having to "brave extremely hazardous conditions".

This is the second ceasefire since the coalition launched its March air campaign against the rebel forces.

A five-day truce in May allowed aid to reach civilians, but UN efforts to prolong it failed.

Meanwhile, 10 suspected Al-Qaeda members were killed in missile strikes by American drones in Mukalla in southeastern Yemen, a local official said.

Saudi-led strikes hit Yemen rebels despite truce
Sanaa (AFP) July 12, 2015 - Saudi-led coalition warplanes bombarded Yemeni rebels Sunday, witnesses said, in a new blow to a UN-proposed truce in the impoverished country where millions are threatened with famine.

Air strikes hit the Shiite Huthi stronghold of Saada in Yemen's north, as well as other rebel positions south of the capital Sanaa and in the southern province of Lahj, residents said.

There were no immediate reports of casualties.

Clashes also raged in the southern port city of Aden between the rebels and fighters allied with exiled President Abedrabbo Mansour Hadi, known as the Popular Resistance.

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

"We have seized control of the area that represents the western entry into Aden," he said, adding that the group "received sophisticated weapons from the coalition."

But the rebels prevented a convoy of desperately needed relief aid travelling from the western port city of Hodeida to enter Aden, said local relief activist Adnan al-Kaf.

Clashes also persisted in the central city of Taez and in the southern Shabwa province, where the Popular Resistance said 21 of its fighters were killed in three days of clashes.

The UN-proposed humanitarian truce technically went into effect at 2059 GMT Friday and is supposed to run until July 17, the last day of the Muslim fasting month of Ramadan.

But the ceasefire, much needed to rush food supplies to a population threatened by famine, has been flouted by strikes conducted by the Saudi-led coalition and fighting on the ground.

The ceasefire was declared after UN chief Ban Ki-moon received assurances from Hadi and the Huthis that it would be respected.

The coalition said it has not received a formal request from Hadi's government to observe a truce, while the rebels said before the ceasefire went into effect they had little hope it would succeed.

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.

Fighting had intensified after the northern rebels overran the capital unopposed in September and went on to advance on several regions, including Aden where Hadi took refuge after escaping house arrest in Sanaa.

Saudi Arabia formed an Arab coalition that launched late March an air campaign to restore Hadi.

The rebels are allied with troops loyal to former president Ali Abdullah Saleh, who stepped down in 2012 following a year of nationwide protests against his three-decade rule.