Saudi-led air raids killed 21 civilians in Yemen's capital Sanaa on Monday morning, relatives of the victims and medics told Reuters, two days after the start of a United Nations-brokered humanitarian truce that Riyadh does not recognise.

"Three missiles targeted the neighbourhood, destroying 15 houses and killing 21 people and wounding 45 others," said a resident.

A Saudi-led Arab coalition has been bombing the Houthi militia and army forces loyal to former president Ali Abdullah Saleh since March 26, aiming to push them back from southern and central areas and restore the country's exiled government.

The Houthis, who are allied to Riyadh's main regional rival Iran, advanced from their northern stronghold a year ago, capturing the capital Sanaa in September and then pushing south early this year, prompting the Saudi-led airstrikes.

More than 3,000 people have been killed in the fighting and air strikes so far, amplifying an existing humanitarian crisis, but the Houthis and Saleh's forces remain embedded across the populated Western side of the country.

The United Nations brokered a pause in the fighting on Friday to allow humanitarian aid to be delivered, but the Saudi-led coalition said it had not been asked by Yemen's exiled President Abd-Rabbu Mansour Hadi, in whose name it is acting, to stop its raids.

Brigadier-General Ahmed al-Asiri, the spokesman of the coalition, was reported by al Sharq al Awsat newspaper as saying there would be no truce because Houthis were not committed to a ceasefire and no UN observers had been deployed on the ground to monitor possible violations.

There have also been reports of fighting in breach of the pause conditions in Aden, Marib and Taiz, the main theaters of battle between local resistence movements, tribes, Islamist militants and the Houthis and Saleh's forces.

A Houthi leader, Saleh al-Samad, described the continued Saudi raids as presenting "a clear challenge to the international community to shoulder its responsibilities and seriously try to stop this aggression".

A United Nations Security Council resolution in April demanded the Houthis and Saleh's forces quit areas they have captured, release prisoners and surrender weapons taken from army units that have been overrun.