Yemen ceasefire efforts gathered pace Sunday after more than six weeks of Saudi-led air strikes, with rebels saying they would respond "positively" and their allies accepting a US-backed truce plan.

The renegade troops, who helped the Shiite Huthi rebels seize much of the country, said they had agreed to the five-day humanitarian truce starting Tuesday that Riyadh has offered.

The rebels did not refer explicitly to the Saudi offer, but expressed "readiness to deal positively with any efforts, calls or measures that would help end the suffering".

Saudi's King Salman will not attend a US-Gulf leaders summit next week due to the ceasefire, instead sending his newly named Crown Prince Mohammed bin Nayef to lead the kingdom's delegation.

Salman will miss the meeting "due to the timing of the summit (and) the scheduled humanitarian ceasefire in Yemen," Saudi's embassy in Washington said in a statement.

Meanwhile in Yemen, a ship chartered by the UN's World Food Programme docked in the western port of Hodeida, bringing fuel the organisation said would help provide "a new humanitarian lifeline for civilians impacted by the conflict".

The MV Amsterdam brought 300,000 litres of fuel and supplies for humanitarian organisations, while a second vessel was due to deliver another 120,000 litres.

WFP Yemen director Purnima Kashyap said the fuel will mean aid can reach the "hundreds of thousands of people in need of urgent food assistance".

The United Nations has expressed deep concern about the civilian death toll from the bombing -- and estimated 1,400 people have died in the conflict since March -- and the humanitarian impact of the air and sea blockade Saudi Arabia and its allies have imposed on Yemen.

Coalition warplanes pounded the Huthis' Saada stronghold in the northern mountains for a second straight night Saturday after declaring the whole province a military target.

- 'Civilians trapped' -

Aid agencies said 70,000 people, including 28,000 children, were fleeing Saada. They called for an immediate ceasefire in a statement signed by 17 organisations.

"There is an urgent need to halt hostilities in order to move humanitarian aid to the country," said Daw Mohamed, Yemen country director for CARE International.

The UN's humanitarian coordinator for Yemen, Johannes van der Klaauw, said civilians were trapped in Saada and warned of the dangers of "indiscriminate bombing of populated areas".

But coalition spokesman Brigadier General Ahmed al-Assiri told AFP planes had avoid bombing civilian targets. "We do not conduct any operation in cities," he said.

Warplanes also launched twin strikes Sunday on the Sanaa residence of ousted president Ali Abdullah Saleh, who is accused of orchestrating the alliance between renegade army units and the rebels.

These troops, who remained loyal to Saleh after he was forced from power in early 2012, played a major part in the Iran-backed rebels' capture of swathes of the country.

"Following mediation from friendly countries to establish a humanitarian truce... we announce our agreement," said Colonel Sharaf Luqman, spokesman for the pro-Saleh forces.

The defectors' bases have been a major target in the coalition campaign in support of exiled President Abedrabbo Mansour Hadi.

The rebels welcomed efforts by "friendly countries to end the aggression and the suffering of the Yemeni people" -- an apparent reference to Russia which has pressed for a halt to the air war.

- Conditional ceasefire -

Saudi Arabia said its ceasefire offer is conditional on the rebels reciprocating and not exploiting it for military advantage.

Saleh's political party, the General People's Congress, said it hoped the proposal would minimise the "impact of the aggression that has burdened the Yemeni people with unprecedented suffering and an unparallelled blockade".

Riyadh said the rebels had crossed a "red line" with deadly shelling of populated border areas of the kingdom last week.

Assiri told AFP that Saudi artillery again pounded positions inside Yemen after rocket fire wounded four women in the kingdom on Sunday.

Saudi-led forces will continue to retaliate against targets over the border until the 2000 GMT Tuesday ceasefire "if they continue to fire their rockets towards our cities, our population," Assiri said.

Clashes also raged between rebel and pro-Hadi forces in the main southern city of Aden, killing civilians and fighters including prominent Hadi loyalist Haitham al-Adani, medics and military sources said.

The newly formed Aden Alliance for Popular Aid said in a statement the city urgently needed food supplies, medicines, field hospitals and fuel, as well as "safe corridors" in disputed areas so the wounded can be evacuated.

Aircraft from coalition partner the United Arab Emirates, meanwhile, airdropped 55 tonnes of humanitarian aid in Aden, state news agency WAM reported.

In Shabwa province farther east, an apparent US drone strike killed four Al-Qaeda suspects, a military official said.