A Saudi-initiated humanitarian truce took effect in Yemen late Tuesday, after 24 hours of intensive bombing by a military coalition led by the kingdom targeting Iran-backed rebels.
But the Saudi-led coalition warned the Shiite Huthi rebels it stood ready to strike back at any violation of the pause that comes after more than six weeks of bombing.
"Absolutely. 11 O'clock", coalition spokesman Brigadier General Ahmed al-Assiri said when asked whether the ceasefire had begun on schedule at 11:00 pm (2000 GMT).
The five-day pause aims to allow deliveries of desperately needed relief supplies, although aid groups have already warned they need more time.
"We are committed to respect this," Assiri said, but the coalition will continue its "intelligence, reconnaissance and surveillance" in case it has to respond.
"We will be ready to react to any violation of the pause," he told AFP. "We are very clear. If they do not respect... we will continue."
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 Mount Noqum, on the eastern outskirts of the rebel-held capital, lasted until midday Tuesday, when a fresh wave of strikes hit the depot, 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 for talks with the rebels, ahead of other meetings this week, possibly in Iran.
"We are not ready to announce a date for talks but that remains the goal," UN spokesman Stephane Dujarric said, adding that "the fact that he got into Sanaa and is meeting with the Huthis today is a sign in the right direction."
- Doubts over ceasefire -
Tensions between Sunni-dominated Saudi Arabia and Shiite Iran have soared since the air war began on March 26.
Riyadh has repeatedly accused Iran of arming and funding the rebels, a charge Tehran denies.
The Huthis have promised to respond "positively" to the truce, and allied troops loyal to ousted president Ali Abdullah Saleh have accepted it.
The truce is the first since the Riyadh-led alliance began its campaign to restore the crumbling authority of exiled President Abedrabbo Mansour Hadi.
The ceasefire has strong backing from Washington, which has said it could be extended.
But doubts have been cast on the initiative.
"There might be a ceasefire but it won't end the conflict," said a Western diplomatic source. "I'd be surprised if it was honoured across Yemen. There will still be skirmishes going on."
The UN has expressed deep concern about the civilian death toll from the Saudi-led bombing as well as the humanitarian impact of an air and sea blockade imposed by the coalition.
It 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 Saudi border, 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 -
Clashes raged in the southern provinces of Abyan, Shabwa and Daleh, as well as Jawf farther north, residents said.
Bloody clashes were also reported 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 late Tuesday ahead of the truce, according to witnesses.
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.
Another drone strike in Shabwa killed three insurgents.
Meanwhile, the coalition denied rebel claims they had shot down a Moroccan F-16 warplane, which crashed in Yemen at the weekend.
Assiri said a technical fault or human error were to blame. The pilot was still missing.