Saudi-backed forces have begun an assault on the key port of Hudaydah after Houthi rebels ignored a final deadline to withdraw, reports say, BBC reports.
Rebel positions are being bombarded from the air and sea, according to regional media.
Aid agencies have warned of a humanitarian catastrophe if the city is attacked, with up to a quarter of a million casualties possible.
The port is the main entry point for humanitarian aid into Yemen.
More than seven million people in the war-torn country are dependent on food aid.
The Saudi-owned Al-Arabiya network reported that the "liberation" of Hudaydah had begun with a large-scale ground assault supported by air and naval cover.
Explosions were heard on the outskirts of the port city, it added.