The Saudi-led coalition bombed the Yemeni capital's airport and the Shiite rebels' northern stronghold on Saturday just hours after proposing a humanitarian ceasefire to start next week.

Hundreds of families fled the rebels' Saada province bastion before a coalition deadline to leave expired on Friday evening, after which warplanes pounded rebel bases and the homes of rebel leaders, witnesses said.

But aid agencies warned that many civilians would not have had time to leave the province by the deadline and urged the coalition to hold back from its threat to treat the whole province as a military target.

After six weeks of air strikes in support of exiled President Abedrabbo Mansour Hadi, the coalition zeroed in on Saada, the rugged heartland of the Huthi Shiite rebels on the Saudi border.

Riyadh has warned that the Iran-backed rebels crossed a "red line" by shelling populated border areas of the kingdom earlier this week.

Coalition warplanes and Saudi artillery pounded rebel chief Abdul Malik al-Huthi's hometown of Marran, witnesses said.

The rebel's Al-Masirah television said Marran and nearby Baqim were hit by more than 160 rockets.

The channel said the coalition had launched more than 27 air raids across the province.

Strikes in Saada city brought down the telecommunication network, residents said.

The coalition also carried out new strikes against the airport in the rebel-held capital Sanaa, which already suffered damaged to its runaways in previous strikes.

The coalition has imposed a tight sea and air blockade in a bid to prevent supplies reaching the rebels and their allies in the security forces.

The rebels have yet to respond to the proposal made by Saudi Foreign Minister Adel al-Jubeir on Friday for a five-day ceasefire beginning on May 12.

"The ceasefire will end should Huthis or their allies not live up to the agreement -- this is a chance for the Huthis to show that they care about their people and they care about the Yemen people," Jubeir said at a joint news conference in Paris with US Secretary of State John Kerry.

Kerry said the ceasefire would take place "provided that the Huthis agree that there will be no bombing, no shooting, no movement of their troops or manoeuvring to reposition for military advantage (and) no movement of heavy weapons".

He stressed that the pause in hostilities was a "renewable commitment" that, if it held, "opens the door to possibility of an extension".

The United Nations has called repeatedly for a ceasefire after weeks of fighting that have killed more than 1,400 people, many of them civilians.

Even before the latest exodus of civilians from Saada, the UN estimated that at least 300,000 had been displaced by the conflict.

More than 800 families took refuge in neighbouring Amran province on Friday, witnesses said. Scores more fled to the capital.

Doctors without Borders (MSF) warned it was "impossible" for the province's entire population to leave in a matter of hours, and called on its Twitter account for the coalition to avoid hitting residential areas.

In Yemen's main southern city Aden, there was no letup in weeks of fighting between the rebels and their allies, and forces loyal to Hadi.

Eight rebels and four loyalists were killed in heavy clashes in the north of the port city, military sources said.

Al-Qaeda, which already had a firm foothold in parts of the south and east, has taken advantage of the conflict to expand its grip seizing a provincial capital, an airport and a series of military bases.

A US drone strike killed three Al-Qaeda militants in Shabwa province east of Aden, a tribal chief said.

Washington has waged a longstanding drone war against Al-Qaeda's Yemen branch, which it regards as the most dangerous, but was forced to withdraw its forward operators from a southern air base as the rebels advanced on Aden.