Saudi-led air strikes and heavy shelling between warring factions shook several cities in Yemen on Saturday, residents said, violating a United Nations humanitarian truce which took effect just before midnight.

The UN-brokered pause in the fighting was meant to last a week to allow aid deliveries to the country's 21 million people who have endured over three months of bombing and civil war.

A coalition of Arab states has been bombing the Iranian-allied Houthi rebel movement - Yemen's dominant force - since late March in a bid to restore to power President Abd-Rabbu Mansour Hadi, who has fled to Riyadh.

Air raids pounded Houthi and Yemeni army units in the capital Sanaa and in the embattled southern cities of Taiz and Aden, where residents also reported intense artillery exchanges between the fighters and local militiamen.

In Aden, one of the country's most deprived and war-torn areas, witnesses said Houthi forces fired mortars and Katyusha rockets toward opposition fighters based in northern areas and around the city's international airport.

Bombing by the Arab alliance and fighting have killed more than 3,000 people since March 26.

The Houthis, who hail from the Shi'ite Muslim sect, and their army allies say their spread throughout the country is part of a revolution against a corrupt government and hardline Sunni Muslim militants which they say are allied to the opposition forces, and they vowed to keep up the fight despite the truce.

"Our security and armed forces maintain their right to fight and hunt down Al Qaeda and Islamic State elements as part of our just defence of our people," Colonel Sharaf Luqman, spokesperson for the Houthi-allied army, said in a statement on Saturday.

All parties in Yemen's conflict had welcomed the announcement of the truce and called for it to be extended.

But the exiled government wants the Houthis to release prisoners and give up land it has seized in battle, while the Houthis say they doubt any calm will last.

"The Saudi aggression, with these violations, confirms that it will persist with its aggression on Yemen and won't carry through on its pledges to the United Nations," a military official was quoted telling the Houthi-controlled state news agency Saba.-Reuters