The UN and more than 20 humanitarian groups have condemned a blockade by the Saudi-led coalition on airports, seaports and land crossings into Yemen, saying it could bring millions of people closer to "starvation and death".
The groups, including CARE, Save the Children and Islamic Relief, said about two-thirds of Yemen's population relies on imported supplies.
They said more than 20 million people need humanitarian assistance, including seven million facing "famine-like" conditions. Food supplies are set to run out within six weeks, and vaccines will last one month.
The groups called for the "immediate opening" of all air and seaports in the statement.
The military coalition tightened its blockade in Yemen this week after a ballistic missile fired by Houthi rebels was intercepted near the Saudi capital.
Hundreds of cars lined the main roads of Yemen's capital after the rebels who control the city ordered fuel stations to close on Wednesday, accusing merchants of taking advantage of the blockade to hike prices. Fuel prices have spiked by 50% since Monday.
Hassan al-Zaydi, a spokesman for the Houthi-run Oil Ministry, said merchants had refused orders to keep prices fixed, prompting authorities to shut the fuel stations down.
A UN official said aid agencies were given no notice of the Saudi decision to shut down all land, air and seaports, and had learned about it from media.
George Khoury, head of the UN Office for the Co-ordination of Humanitarian Affairs in Yemen, said on Wednesday: "We want to be crystal clear to the international community - any disruption will have catastrophic consequence on the lives of hundreds of thousands people and children."
Later in the day, the UN humanitarian chief, Mark Lowcock, warned that if the blockade continued, Yemen would face "the largest famine the world has seen for many decades, with millions of victims".
After briefing the UN Security Council behind closed doors, he called for a resumption of air flights into Yemen for the United Nations and its humanitarian partners and said there must be immediate access to all ports, especially for food, fuel, medicine and other essential supplies.
Every month, at least seven million people depend on life-saving UN aid.
The Saudi-led coalition has been at war with the Houthis, a Shiite group supported by Iran, since March 2015. Saudi Arabia blamed Saturday's missile strike on Tehran, which supports the rebels but denies arming them.