Thousands of civilians will die, including many children, unless a Saudi-led coalition fully lifts a blockade that has "choked off" aid supplies to Yemen, the heads of three UN agencies said on Thursday.
"Together, we issue another urgent appeal for the coalition to permit entry of lifesaving supplies to Yemen in response to what is now the worst humanitarian crisis in the world. The supplies, which include medicines, vaccines and food, are essential to staving off disease and starvation", a joint statement said.
"Without them, untold thousands of innocent victims, among them many children, will die", it added.
The stark warning from World Health Organisation chief Tedros Adhanom Ghebreyesus, Unicef director Anthony Lake and World Food Programme director David Beasley, came 10 days after the Saudi-led military coalition shut down Yemen's sea and air ports as well as borders.
The Saudi move was in response to a missile attack by the Iran-backed Houthi rebels near Riyadh.
The coalition has partially eased some restrictions on ports controlled by the Yemeni government, which is Saudi Arabia's ally in the country's devastating civil war.
But rebel-held entry points, notably the crucial Red Sea port of Hodeida, remain shut.
Riyadh has said that tighter restrictions must be put in place at Hodeida before aid can resume flowing, a condition the UN has rejected.
"All of the country's ports -- including those in areas held by the opposition -- should be reopened without delay," the statement said.
"The clock is ticking and stocks of medical, food and other humanitarian supplies are already running low. The cost of this blockade is being measured in the number of lives that are lost", it added.
Saudi Arabia and its allies intervened in Yemen in March 2015 to push back the Houthi rebels who control the capital Sanaa, and restore the government of President Abedrabbo Mansour Hadi to power.