UNITED NATIONS (Reuters) - A Saudi Arabia-led coalition and Houthi rebels fighting in Yemen are both restricting humanitarian aid access in the impoverished country where more than 80 percent of people need help, United Nations aid chief Stephen O’Brien said on Tuesday.

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The Saudi coalition began a military campaign in March of last year to prevent Iran-allied Houthi rebels from taking complete control of Yemen. The Houthis and forces loyal to former Yemen President Ali Abdullah Saleh, accuse the coalition of a war of aggression.

O’Brien said the Houthis were inconsistent in allowing access and movement of humanitarian goods and personnel, while a recent warning by Saudi Arabia about the safety of aid workers in “Houthi-controlled areas” caused delays to key missions.

“The parties to the conflict have a duty of care in the conduct of military operations to protect all civilian persons and objects - including humanitarian and health care workers and facilities - against attack,” he told the U.N. Security Council.

“I remind all parties of their obligations under international humanitarian law to facilitate humanitarian access to all areas of Yemen,” O’Brien said.

Saudi Arabia, which is leading air strikes against the Houthis and their allies in Yemen, warned the United Nations and international aid groups to protect staff by removing them from areas near rebel military bases.

“U.N. agencies and NGO partners are delivering assistance under extraordinarily difficult and dangerous circumstances,” O’Brien said. “Just this last Sunday, a Saudi-led coalition airstrike hit a building 200 meters away from the Diplomatic Transit Facility, accommodating U.N. and diplomatic personnel.”

He said more than 6,000 people had been killed in the past year, of which about half were civilians. He said more than 700 children had been killed and some 1,000 injured.

U.N. sanctions monitors said in a report last month that the Saudi-led coalition and the Houthi rebels have targeted civilians and that some of the attacks could be a crimes against humanity.

Yemen relies almost solely on imports, but the conflict has slowed shipments to a trickle. O’Brien said a World Food Programme ship carrying humanitarian supplies and headed to the Yemeni port of Hodeidah was diverted by coalition forces on Feb. 11 to the Saudi port of Jizan.

He said a U.N. verification and inspection mechanism for shipments had now been launched in a bid to boost commercial imports and that the United Nations has asked the Saudi coalition and Yemeni government to appoint representatives to a steering committee by Feb. 22.