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 set by the government to withdraw by midnight (21:00 GMT on Tuesday).
On Tuesday, the exiled government of President Abdrabbuh Mansour Hadi said efforts persuade the rebels to withdraw had been exhausted.
The Saudi-owned Al Arabiya network reported early on Wednesday that the “liberation” of Hudaydah had begun with a large-scale ground assault supported by air and naval cover from a Saudi-led multinational coalition.