Saudi-backed forces have begun an assault on the key Yemeni port of Hudaydah, which is held by rebels.
Hudaydah is the main point of entry for aid in Yemen’s three-year civil war and agencies have warned of a humanitarian catastrophe if it is attacked.
More than seven million people in the war-torn country are dependent on aid.
Bombing started after Iranian-backed Houthi rebels ignored a deadline set by the exiled government to withdraw by midnight (21:00 GMT on Tuesday).
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 liberation of Hudaydah port is a turning point in our struggle to recapture Yemen from the militias that hijacked it to serve foreign agendas,” the government of President Abdrabbuh Mansour Hadi said.
The Saudi-backed coalition and the government have accused the rebels of using the port to smuggle in Iranian weapons, which the rebels deny.
The United Arab Emirates 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.