Saudi-backed government forces have begun an assault on the key Yemeni port of Hudaydah, which is held by rebels.
BBC: The port is the main point of entry for aid for people in rebel-held areas and agencies have warned of a humanitarian catastrophe if it is attacked.
About eight million people in the war-torn country are at risk of starvation.
Bombing started after Iranian-backed Houthi rebels ignored a deadline set by the government to withdraw by midnight (21:00 GMT on Tuesday).
The conflict has raged since late 2014, when the Houthis and allied forces seized north-western parts of Yemen, including the capital Sanaa, and eventually forced President Abdrabbuh Mansour Hadi to flee abroad.
Alarmed by the rise of a group they saw as an Iranian proxy, Saudi Arabia and eight other Sunni Arab states launched a military campaign in March 2015 to restore Mr Hadi’s government.
The conflict has created the world’s largest food emergency.
The Saudi-led coalition has accused the rebels of using Hudaydah to smuggle in Iranian weapons. Both Iran and the rebels deny this.
But the port is a lifeline for the majority of Yemen’s population and the UN had been trying to get parties to the conflict to reach a deal that would avert an attack.
An estimated 600,000 people live in the area.
On Friday, the UN Humanitarian Co-ordinator for Yemen Lise Grande warned: “In a prolonged worst case, we fear that as many as 250,000 people may lose everything – even their lives.”
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