A Saudi-initiated humanitarian truce took effect in Yemen, 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 violations of the pause that comes after more than six weeks of bombing.
Shortly after the truce began guns fell relatively silent in the southern city of Aden, which has seen heavy combat between pro-government forces and the Huthis and their allies.
Lahj and Abyan in Yemen's south were also calmer, while rebels moved troops to reinforce positions at Marib, near the capital Sanaa, and Dhaleh, witnesses said.
Sanaa was calm about an hour after the ceasefire started.
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," Brigadier General Ahmed al-Assiri said, warning the rebels to do the same.
"We will be ready to react to any violation of the pause," he told AFP.
Explosions at an arms depot in the capital hit by 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".
The newly appointed UN envoy to Yemen, Ismail Ould Cheikh Ahmed, arrived in Sanaa for talks on restarting a collapsed political dialogue.
"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.
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The UN Security Council welcomed the pause and expressed grave concern for the "severe humanitarian consequences" of the months-long violence in Yemen.
"All parties will need to transparently and reliably suspend military operations" for the humanitarian pause to hold, the council said.
The pause is the first since the Riyadh-led alliance began its campaign to restore the crumbling authority of exiled President Abedrabbo Mansour Hadi.
But doubts have been cast on the initiative, which has strong backing from Washington.
"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.
UN agencies were preparing a massive aid operation as soon as the truce began.
Since last week, 12 people have been killed on the Saudi side of the border from rebel mortar and rocket fire, which continued before Tuesday's ceasefire, Assiri said.
The Huthis, 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.
- 'Catastrophic' humanitarian situation -
Clashes raged in southern provinces and other parts of the country before the ceasefire, while coalition air strikes hit 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.
"We hope the truce would last longer, and become permanent. And we hope all sides respect it," Red Cross 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.
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.