A peace conference for Yemen expected to be held next week in Geneva was put on hold on Monday as fighting resumed in the Arab country.
UN Secretary-General Ban Ki-moon was expected to announce the conference to re-launch talks between all parties to the Yemen conflict.
But UN spokesman Farhan Haq said "part of the problem is that the fighting has once more resumed."
"We want the fighting decisively stopped and then we can get about to organize and invite people to the conference."
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"Until that happens, some of that remains a bit hypothetical," he said.
The Saudi-led coalition resumed air strikes on Yemen after a five-day humanitarian pause ended late Sunday, ignoring UN calls for an extension.
Ban regrets that there was no extension and "calls on all parties to create the conditions leading to a permanent ceasefire," said the UN spokesman.
The coalition launched an air war against Yemen on March 26 to push back an advance by Iranian-backed Shiite Huthi rebels who seized Sanaa and Aden, forcing the president to flee into exile.
UN envoy Ismail Ould Cheikh Ahmed, who was in Riyadh for talks, had called for the humanitarian truce to be prolonged for an extra five days.
The Huthis and pro-government forces fought battles across southern Yemen on Saturday, but the truce had largely held since Tuesday.