Warring sides must let more aid get through to 8.4 million people who are "a step away from famine" in Yemen, a senior UN official said on Monday.
A Saudi-led military coalition fighting the Iran-aligned Houthi movement in Yemen's civil war blockaded ports last month after a missile was fired towards Riyadh.
Jamie McGoldrick, the Humanitarian Coordinator for Yemen, said the blockade has since been eased, but the situation remained dire.
"The continuing blockade of ports is limiting supplies of fuel, food and medicines; dramatically increasing the number of vulnerable people who need help," McGoldrick said in a statement.
"The lives of millions of people, including 8.4 million Yemenis who are a step away from famine, hinge on our ability to continue our operations and to provide health, safe water, food, shelter and nutrition support," he added.
That marked an increase from past UN estimates of around 8 million people on the brink of famine.
The coalition accuses Iran of sending weapons to its Houthi allies, including missile parts, through Yemen's main Hodeidah port, were most food supplies enter.
Saudi state television said on Monday a UN delegation of experts has arrived in Riyadh to meet the coalition and the Yemeni government the coalition supports "to prevent the transfer of weapons and rockets to Houthis".
Iran has denied supplying the Houthis with weapons, saying the US and Saudi allegations are "baseless and unfounded".
The United Nations says food shortages caused by the warring parties blocking supplies has created the world's worst humanitarian crisis.
The Saudis intervened in neighboring Yemen in 2015 after the Houthis advanced on the southern port city of Aden and forced President Abd-Rabbu Mansour Hadi and his government into exile.
The conflict has killed more than 10,000 people, displaced more than two million and triggered a cholera epidemic that has infected about one million people.
REUTERS