Saudi Arabia, which is leading air strikes against rebels in neighbouring Yemen, has warned the United Nations and international aid groups to protect staff by removing them from areas held by Houthi rebels.
A short note sent by the Saudi Embassy in London on Friday said the intention was to "protect the international organisations and their employees", presumably from coalition air strikes.
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Saudi Arabia leads a coalition of nine Arab countries that began a military campaign in March to prevent Houthi rebels, whom it sees as a proxy for Iran, from taking complete control of Yemen.
The Saudi ambassador to the UN in New York, Abdallah al-Mouallimi, told Reuters news agency that Riyadh sent the letter because "we're just concerned for the safety of the UN staff and their humanitarian agencies".
"We want them to go away from areas that are obvious targets," he said.
UN aid chief Stephen O'Brien acknowledged receipt of the note and said the humanitarian community would continue to deliver aid across Yemen impartially on the basis of need.
He reminded Saudi Arabia of obligations under international humanitarian law to facilitate access for aid.
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The UN Security Council is due to discuss the humanitarian situation in Yemen on Tuesday at the request of Russia, diplomats said.
The Houthis and their allies, forces loyal to former Yemen President Ali Abdullah Saleh, accuse the coalition of launching a war of aggression.
Nearly 6,000 people have been killed since the coalition entered the conflict in March, almost half of them civilians.
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