Saudi-backed government forces have begun an assault on the key Yemeni port of Hudaydah, which is held by rebels.
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 to withdraw from Hudaydah by midnight (21:00 GMT on Tuesday).
It is the first time the Saudi-led coalition of mostly Gulf states has tried to capture such a well-defended major city in Yemen.