By Michelle Nichols

UNITED NATIONS, July 28 (Reuters) - United Nations aid chief Stephen O'Brien said on Tuesday none of the warring parties in Yemen had observed a humanitarian pause in fighting announced by the Saudi Arabia-led coalition forces and accused both sides of failing to respect international law.

The Arab coalition, which has been bombing Iran-allied Houthi rebels in Yemen for four months, had said a five-day truce would start on Sunday to allow in emergency aid amid severe shortages of fuel, food and medicine.

"A humanitarian pause announced over the weekend has not been respected by any party to the conflict with air strikes and ground fighting reported in eight governorates," O'Brien told the 15-member U.N. Security Council.

"Parties to the conflict continue to fail to meet their responsibilities under international humanitarian and international human rights laws," he said. "We continue to witness the death and injury of civilians."

Saudi Arabia's U.N. Ambassador Abdallah Al-Mouallimi told reporters the coalition was not targeting civilians, and any reports of civilian casualties were fully investigated.

O'Brien said health facilities in Yemen reported that more than 4,000 people had been killed and some 19,800 injured during the four month conflict.

Warplanes attacked Houthi militia in control of Yemen's largest air base north of the port city of Aden on Tuesday.

The violence prolongs a conflict rooted in political strains that spread across the Arabian Peninsula country last year, when the Houthis seized the capital Sanaa and pushed aside President Abd-Rabbu Mansour Hadi, a U.S. ally, who fled to Saudi Arabia.

Yemen relies on imports, but a near-total blockade led by Saudi Arabia has slowed shipments to the war-torn Arabian Peninsula country to a trickle. The Arab coalition is inspecting shipments in a bid to thwart any arms deliveries to the Houthis.

"A light U.N.-led inspections mechanism enabling the flow of commercial imports to increase has long been proposed and is still urgently needed. Negotiations continue," said O'Brien.

In anticipation of a five-day pause the U.N. and partners had developed a plan to reach an additional three million people with food, water, sanitation, healthcare, and treatment for 2,200 children under five with malnutrition, he said.

"That plan is live and ready to go now if only we could get a pause to stick," he added.

Both sides had largely observed a five-day, U.N.-brokered humanitarian truce in May, but a second truce earlier this month also failed.

(Reporting by Michelle Nichols; Editing by Tom Brown) ((michelle.nichols@thomsonreuters.com; +1 646 223 6117; Reuters Messaging: michelle.nichols.reuters.com@reuters.net; Twitter: @michellenichols))

Keywords: YEMEN SECURITY/UN