Saudi-led warplanes pounded rebel targets in Yemen's capital Tuesday for the first time since the end of a five-day ceasefire, as hopes of a political breakthrough in the conflict fade.

A UN-sponsored peace conference for Yemen originally set for next week has been put on hold due to the resumption of fighting, which the United Nations says has killed some 1,850 people and displaced more than half a million.

A three-day conference wrapped up Tuesday in Riyadh and publicly backed the government of exiled Yemeni President Abedrabbo Mansour Hadi, although rebel groups boycotted the meeting.

The Saudi-led coalition resumed its bombing campaign early on Monday with raids on second city Aden, accusing the rebels of violating the temporary ceasefire that expired at the weekend.

Tuesday's raids in the capital hit the presidential palace complex and several bases of troops loyal to ousted strongman Ali Abdullah Saleh, allied with the Shiite Huthi rebels.

Witnesses said the targets included the Republican Guard missile brigade base in Fajj Attan, south Sanaa, where strikes last month set off a chain of explosions that killed 38 civilians.

Coalition aircraft also hit air defence and coastguard bases in Hodeida province on the Red Sea coast, witnesses said.

Residents reported raids in the central province of Taez, and in Daleh and Aden in the south.

The United Nations and aid agencies had pleaded for an extension of the ceasefire, the first pause in the bombing campaign launched in March in a bid to restore the authority of Hadi.

But coalition spokesman Brigadier General Ahmed al-Assiri accused the rebels of abusing the truce.

"They did not respect the humanitarian pause. That's why we do what is necessary to be done," he told AFP.

- UN talks postponed -

A planned, UN-brokered meeting of Yemeni political groups in Geneva has been suspended indefinitely.

"Part of the problem is that the fighting has once more resumed," said UN spokesman Farhan Haq.

"We want the fighting decisively stopped and then we can get about to organise and invite people to the conference."

UN Secretary-General Ban Ki-moon said he regretted that the five-day humanitarian truce had not been extended and "calls on all parties to create the conditions leading to a permanent ceasefire," said the UN spokesman.

US State Department spokesman Jeffrey Rathke praised Saudi Arabia for not retaliating to what he described as Huthi violations during the ceasefire.

"The Huthis shelled Saudi territory numerous times during the pause. And we think the Saudis exercised restraint during this pause, which enabled food, fuel and humanitarian aid to reach vulnerable Yemeni citizens," he said.

Rathke said that political dialogue will ultimately be "the only solution to the crisis," calling on the Huthis to "indicate their readiness and their willingness to come back to the table as part of a UN-led process".

- Rebels 'hijacked' aid -

Iran, a key ally of the Shiite Huthi rebels, called for an end to the "barbaric" Saudi bombing, and said Riyadh was not a suitable location for peace talks.

Ali Akbar Velayati, foreign affairs adviser to Iranian supreme leader Ayatollah Ali Khamenei, told reporters in Beirut that there must be an "end to barbaric Saudi raids that target innocent Yemeni civilians".

The United Nations, citing Yemen health services, says that as of May 15, some 1,850 people had died and 7,394 were injured in the conflict since late March.

The ceasefire allowed supplies of petrol and food to be delivered to Yemen but anti-Huthi groups accuse the rebels of confiscating the aid.

"The truce has only served the aims of the militia, which has increased its readiness and stocked fuel through aid that arrived in their areas of control," said a statement from pro-Hadi forces in the central city of Taez.

A coalition spokesman said the rebels had "hijacked" food and fuel aid.