CAIRO — The top United Nations relief official said Thursday that grain to feed 3.7 million Yemenis is stranded and possibly rotting in warehouses in the front-line port city of Hudaydah, while nearly 10 million Yemenis verge on famine.

The fate of the grain supply has become a symbol of faltering international efforts to implement a limited truce in Hudaydah between Houthi rebels and the Saudi-led coalition, which was signed in December.

Distribution of the grain is a matter of life-or-death urgency for suffering Yemenis, in a war that began nearly four years ago and has become what the United Nations has called the world’s worst man-made humanitarian crisis.

Two out of three Yemenis do not know where their next meal will come from.

Mark Lowcock, the United Nations relief coordinator, said in a statement that the Houthis had refused permission to United Nations officials to reach the Red Sea Mills, where the World Food Program, the organization’s anti-hunger agency, has 51,000 tons of wheat in storage. That is a quarter of the agency’s stock in Yemen.