A Saudi-set humanitarian truce was due to begin in Yemen late Tuesday, even as a military coalition led by the kingdom carried out new air strikes on the capital Sanaa.

The five-day ceasefire -- scheduled to start at 11:00 pm (2000 GMT) -- aims to allow deliveries of desperately needed relief supplies, although aid groups have already warned they need more time.

Explosions at an arms depot in the capital hit by Saudi-led coalition strikes since Monday killed at least 69 people and wounded 250, mostly civilians, an official said.

The blasts at a military base at Mount Noqum, on the eastern outskirts of the rebel-held capital, lasted until midday Tuesday after coalition strikes sent debris crashing into a residential area.

The depot was bombed again on Tuesday, an AFP correspondent said.

UNESCO director general Irina Bokova condemned "severe damage" caused to heritage sites in Yemen, such as Sanaa's old city, during intense bombing.

She called on "all parties to keep cultural heritage out of the conflict".

Quiet returned to Sanaa as the newly appointed UN envoy to Yemen, Ismail Ould Cheikh Ahmed, arrived after touring Gulf members of the coalition.

"We are convinced that dialogue is the only way to solve the Yemeni problem," the rebel-controlled Saba news agency quoted him as saying.

- UN concern at toll -

The Mauritanian diplomat was appointed late last month following the resignation of his predecessor Jamal Benomar, who had lost the support of Gulf states.

Tensions between Sunni-dominated Saudi Arabia and Shiite Iran have soared since the launch of the aerial campaign on March 26.

Riyadh has repeatedly accused Iran of arming and funding anti-government Huthi rebels, a charge Tehran denies.

The Huthis have promised to respond "positively" to the planned truce, and allied troops loyal to ousted president Ali Abdullah Saleh have accepted the proposed ceasefire.

The truce would be the first since the Riyadh-led alliance launched an air war aimed at restoring the crumbling authority of exiled President Abedrabbo Mansour Hadi.

The planned ceasefire has strong backing from Washington, which has said it could be extended.

The United Nations has expressed deep concern about the civilian death toll from the Saudi-led bombing campaign as well as the humanitarian impact of an air and sea blockade imposed by the coalition on Yemen.

The UN says more than 1,500 people have died in the conflict since late March.

The Huthis, who hail from Yemen's mountainous north near the border with Saudi Arabia, overran Sanaa in September and extended their control to other regions.

Hadi fled to Riyadh as the rebels closed in on his refuge in the main southern city of Aden, where clashes between his opponents and supporters have shown no sign of relenting.

- 'Catastrophic' humanitarian situation -

Six people were killed and 53 were wounded Monday in fighting in the port city, health authorities said. Civilians were among the casualties.

Clashes also raged in the southern provinces of Abyan, Shabwa and Daleh, as well as Jawf farther north, residents said.

They also reported bloody clashes in the central province of Baida, as well as coalition air strikes on a Huthi-held camp in Marib province, east of Sanaa.

Air raids rocked the Huthi stronghold province of Saada and third city Taez, also hit by deadly fighting, residents said.

The Red Cross said a longer ceasefire was needed.

"We hope the truce would last longer, and become permanent. And we hope all sides respect it," spokesman Adnan Hizam said, lamenting a "catastrophic" humanitarian situation.

Human Rights Watch warned the Huthis had intensified recruitment of children in violation of international law.

Commanders of the rebels and other groups "should stop using children or risk prosecution for war crimes", HRW said.

It said Islamist and tribal militias as well as Al-Qaeda were also recruiting children.

Al-Qaeda has exploited the growing turmoil to consolidate its grip on Yemen's southeastern province of Hadramawt.

A provincial commander, Maamoun Hatem who headed Al-Qaeda in the central province of Ibb, was among four militants killed Monday in an apparent US drone attack in Hadramawt, an official said.

Meanwhile, the coalition denied rebel claims they had shot down a Moroccan F-16 warplane, which crashed in Yemen on Sunday.

"We are definitely sure it wasn't shot down," said Brigadier General Ahmed al-Assiri.