The United Nations said more than half a million people have been displaced in the conflict in Yemen, whose capital was bombarded Tuesday for the first time since a ceasefire ended.

Hopes faded for a political breakthrough in the two-month conflict as a U.N.-sponsored peace conference originally set for next week was put on hold due to the resumption of fighting.

President Barack Obama tried to reassure America's Gulf allies Thursday that engaging with Iran would not come at their expense, at a Camp David summit that proved short on concrete outcomes.

Pledging to counter external threats to Gulf states amid Iran's growing role in the region, Obama said his security commitment to the decades-old allies was "ironclad."  

The United Nations cultural agency on Tuesday condemned "severe damage" caused to heritage sites in Yemen, such as Sanaa's old city, during intense bombing of the rebel-held capital.

"I condemn these destructions and I call on all parties to keep cultural heritage out of the conflict," said UNESCO director general Irina Bokova.

A Saudi-set humanitarian truce is poised to take effect in Yemen late Tuesday, after 24 hours of intensive bombing by a military coalition led by the kingdom targeting Iran-backed rebels.

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.

The Saudi-led coalition bombed the Yemeni capital's airport and the Shiite rebels' northern stronghold on Saturday just hours after proposing a humanitarian ceasefire to start next week.

Hundreds of families fled the rebels' Saada province bastion before a coalition deadline to leave expired on Friday evening, after which warplanes pounded rebel bases and the homes of rebel leaders, witnesses said.

Rebel fighters on Wednesday shot dead the commander of President Abedrabbo Mansour Hadi's loyalist forces in southern Yemen, a military official said. 

General Ali Nasser Hadi was killed as he led forces fighting Huthi Shiite rebels in the Tawahi district of the port city of Aden, the official told Agence France Presse. 

The United States defended its supplies of cluster munitions Sunday, pointing to restrictions on the transfers, after Human Rights Watch said the bombs had been used by the Saudi-led coalition in Yemen.

HRW said it had gathered photographs, video and other evidence indicating that cluster munitions had been used in coalition air strikes against the Huthi rebel stronghold of Saada province in Yemen's northern mountains in recent weeks.

A new U.N. envoy was looking to kickstart peace talks in Yemen as battles raged Sunday between Iran-backed rebels and pro-government forces a month after the launch of Saudi-led air strikes.

The Shiite Huthi rebels, who have overrun large parts of the country and forced President Abedrabbo Mansour Hadi to flee abroad, have demanded an end to the air war as a condition for U.N.-sponsored talks.

A newly announced division of the Islamic State group calling itself the "Green Brigade" on Thursday claimed a bombing in Yemen that it said had killed five Shiite rebels.

In a message on Twitter, the group said it had struck a vehicle of the Huthi rebels in Yarim in the central province of Ibb on Wednesday.

Rebels in Yemen have released the defense minister, a brother of the exiled president and another general held since late March, a source close to mediators said Wednesday.

"Defence Minister General Mahmud al-Subaihi, General Nasser Mansour Hadi (the president's brother), and (army) General Faisal Rajab have been freed," the source said.