Saudi-backed forces have begun an assault on the key port of Hudaydah after Houthi rebels ignored a final deadline to withdraw, reports say.
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.
The Yemeni government of President Abdrabbuh Mansour Hadi said in a statement that all political means to persuade the rebels to withdraw had been exhausted.
The United Arab Emirates (UAE), part of the Saudi-led coalition that backs Mr Hadi, had earlier given the Houthis a final ultimatum to withdraw or face imminent attack.
UAE Junior Foreign Minister Anwar Gargash told the BBC that the coalition had run out of patience with diplomatic efforts after a previous 48-hour deadline expired.
He said the coalition wanted the UN to take control of the port but it was prepared to take military action if the Houthis refused to withdraw.
The civil war in Yemen has killed about 10,000 people over the past three years and created what the UN says is the world's worst humanitarian disaster.
A Saudi-led multinational coalition intervened in the conflict in Yemen in March 2015 as forces loyal to President Hadi battled the Houthi movement, which champions Yemen's Zaidi Shia Muslim minority.