* Five-day humanitarian truce largely holding but some skirmishes

* First UN aid flights land during pause, deliver shelter items

* Fuel critical to run hospital generators, provide clean water

(adds landing of first UN aid flights)

By Stephanie Nebehay

GENEVA, May 15 (Reuters) - The United Nations on Friday urged Saudi Arabia and its allies to ease inspections of Yemen-bound cargo, to accelerate imports of vital aid that is being held up despite a humanitarian truce.

The aim is to get supplies to 2.5 million Yemenis deprived of food, fuel and medicine due to fighting by an Iranian-allied militia and air strikes against them by a Saudi-led coalition, violence that has killed at least 1,600 people, U.N. humanitarian coordinator Johannes van der Klaauw said.

The first U.N. aid flights landed in Yemen on Friday since a five-day humanitarian pause was declared on Tuesday night. Two of a planned six cargo planes, which will take a total of 150 tonnes of shelter and relief items, arrived in Sanaa, the U.N. refugee agency said.

A tightly enforced inspection regime has hindered delivery of humanitarian supplies, while roadblocks and continued sporadic fighting are holding up distribution inside the country, van der Klaauw told a Geneva news briefing via audio link from Sanaa.

"The arms embargo and its inspection regime results in commercial goods, be it by air or ship, no longer reaching the country," he said, calling for inspections to be sped up so humanitarian goods, including "food and other life-sustaining necessities can resume".

"At the moment, the pause is globally holding, though I have to say every day we see skirmishes, localised and of short duration, but we still see that there is military activity," van der Klaauw said, citing clashes in Aden and Taiz.

"We also face the roadblocks which prevent not only civilians from moving during this pause but also us from providing essential aid."

He called on all parties to the conflict to abide by pledges of safe passage for humanitarian staff and supplies. "We hope that the pause can be extended, of course in the end it should result in a ceasefire and a political agreement."

The United Nations has been able to import 430,000 litres of fuel in the past week, roughly one-tenth of monthly needs, said Trond Jensen, head of the U.N. Office for the Coordination of Humanitarian Affairs in Yemen.

"It has been difficult to reach our warehouses. I know there has been medical supplies stuck in Aden, 50 tonnes of it at least, there has been food and other supplies that we haven't been able to access," Jensen said.

(Reporting by Stephanie Nebehay; Editing by Robin Pomeroy) ((Stephanie.Nebehay@thomsonreuters.com; +41 22 733 3831; Reuters Messaging: stephanie.nebehay.thomsonreuters.com@reuters.net; twitter @StephNebehay))

Keywords: YEMEN SECURITY/UN