SANAA (AFP) -- Efforts for a ceasefire in Yemen after more than six weeks of Saudi-led air strikes gathered pace Sunday 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 that Riyadh has offered starting from Tuesday.

The rebels themselves made no explicit reference to the Saudi offer but expressed "readiness to deal positively with any efforts, calls or measures that would help end the suffering."

The truce moves came as the United Nations expressed deep concern about the civilian death toll from the bombing campaign and the humanitarian impact of the air and sea blockade that Saudi Arabia and its allies have imposed on its impoverished neighbour.

Coalition warplanes pounded the rebels' stronghold of Saada in the northern mountains for a second straight night on Saturday after declaring the whole province a military target despite aid agency pleas to spare trapped civilians.

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

The renegade units, 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 beyond their stronghold in the mainly Shiite northern highlands.

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

US-backed ceasefire

The defectors' bases have been a major target of the coalition air campaign in support of exiled President Abedrabbo Mansour Hadi that the United Nations says has killed more than 1,400 people, many of them civilians.

The rebels welcomed efforts by "friendly countries to end the aggression and the suffering of the Yemeni people."

That was an apparent reference to Russia, which unsuccessfully put a ceasefire proposal to the UN Security Council earlier this month and has kept up its drive for a halt to the air war.

Saudi Arabia has stressed that its ceasefire offer is conditional on its being reciprocated by the rebels and not exploited for military advantage.

The offer has been given strong backing by US Secretary of State John Kerry who said that the truce could be extended provided the rebels did not abuse it.

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".

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

The ousted strongman, who ruled in Sanaa for more than three decades, was not believed to have been in the capital at the time of the twin strikes on his residence early on Sunday.

Civilians trapped

The UN Security Council has imposed sanctions on both Saleh and his son for their support for the rebels and their undermining of the transition since his ouster following a bloody year-long uprising.

Coalition aircraft carried out intensive air strikes on Saada province for a second straight night after giving civilians until Friday evening to flee.

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

Residents reported at least 15 raids across the province. Rebel chief Abdul Malik al-Huthi's home town of Marran was again among the targets.

Aid agencies warned that large numbers of civilians remain trapped in the province unable to find transport to leave.

The United Nations has said that the fighting and the coalition blockade have led to a rapidly worsening shortage of fuel that is also preventing the distribution of desperately needed aid.

Doctors without Borders (MSF) said it had been "impossible" for Saada's entire population to leave in just hours, and called on the coalition to avoid hitting residential areas.