Food aid in a warehouse on the frontlines of the Yemen war is at risk of rotting, the United Nations said on Monday, leaving millions of Yemenis without access to life-saving sustenance.

The Red Sea Mills silos, located in the western port city of Hodeida, are believed to contain enough grain to feed several million people for a month.

But the granary has remained off-limits to aid organisations for months.

"The World Food Programme (WFP) grain stored in the mills — enough to feed 3.7 million people for a month —has been inaccessible for over five months and is at risk of rotting," read a joint statement by the UN aid chief and special envoy for Yemen.

Read more:Over 7 million Yemeni children face food insecurity: UNICEF

"We emphasise that ensuring access to the mills is a shared responsibility among the parties to the conflict in Yemen."

Hodeida, and its food silos, have been in the hands of Yemen's Houthi rebels since 2014, when the insurgents staged a takeover of large swathes of Yemeni territories.

The coup prompted the military intervention of Saudi Arabia and its allies the following year on behalf of the embattled government, triggering what the UN calls the world's worst humanitarian crisis. More than 10 million Yemenis stand at the brink of starvation.

UN Yemen envoy Martin Griffiths, who in December secured a ceasefire agreement for Hodeida between the Iran-backed rebels and Saudi-led coalition, and UN aid chief Mark Lowcock on Monday said the rebels had made "efforts to re-open the road leading to the mills" in the joint statement.

On Thursday, Lowcock issued a public plea to the Houthis to allow relief groups to cross front lines to reach the Red Sea Mills, warning the remaining grain could spoil.

The Yemen war has killed around 10,000 people since 2015, according to the World Health Organisation. Other rights groups estimate the toll is significantly higher.