By Suleiman Al-Khalidi

AMMAN (Reuters) - Only a lasting ceasefire in Yemen will allow aid workers to fully address the country's "monumental" humanitarian challenge after six weeks of air strikes and conflict, a senior United Nations official said on Friday.

Saudi Arabia has offered a five-day pause in hostilities, on condition that its Houthi foes also stop fighting. [ID:L5N0XY3GP] But there has been no sign of a let-up and Riyadh said on Friday it would expand air strikes in northern Yemen.

U.N. coordinator for Yemen Paulo Lembo said a military pause would allow U.N. agencies to increase relief for tens of thousands of displaced people and tackle shortages of food, medicine, water and fuel.

"I am optimistic it will come into being in the near future and the UN's international system is already prepared to respond to this development," Lembo said. "It will have a very positive consequences on our ability to expand our response."

But only a long-term truce would make a real difference in a conflict where "the scale of the humanitarian challenge we have is not less than monumental", he told Reuters in interview.

"A five day truce would not be sufficient to cover the daunting humanitarian needs of the country. We hope that this soon will transform into a permanent truce," Lembo said.

International concern over Yemen's plight has grown as more than 1,300 people have been killed, infrastructure has been hit and an air and sea blockade imposed by Saudi Arabia and its Arab allies has complicated relief efforts in the Arabian Peninsula's poorest country, which imports 90 percent of its food.

"The situation is tragic and it evolves from bad to worse hour by hour," Lembo said, pointing to fragmentation of warring factions and intensified fighting in the southern city of Aden.

Lembo said a U.N.-chartered vessel with a large cargo of fuel was waiting off Yemen's Red Sea port of Hodaida, and others were being prepared to ferry aid and fuel from nearby Dijbouti.

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"This vessel could first of all unload immediately a consignment of fuel that is much needed for us. There are several vessels that will be enough to expand our operations," Lembo added.

A humanitarian truce would allow the U.N. to expand the work of a 600-strong local team to support those in need "irrespective of their political affiliation or location across the country", Lembo said.

Despite the war affecting the key Sanaa-Taiz-Aden land route, over the last three weeks emergency food was delivered to at least 1.2 million people by the UN's World Food Programme.

"This was in spite of all the risk with all the challenges with all the rescue routes that have been fragmented or blocked by militia groups," Lembo said.

(Reporting by Suleiman Al-Khalidi; Editing by Dominic Evans)