Yemeni forces backed by the Saudi-led coalition gathered near the outskirts of the western port city of Al Hudaydah, Yemen, on Tuesday.

DUBAI, United Arab Emirates — A coalition led by Saudi Arabia and the United Arab Emirates began an assault early Wednesday on the Yemeni city of Al Hudaydah, seeking to oust Iranian-backed Houthi rebels who control the strategic port, the Yemeni government said.

Several days of intensive diplomacy failed to stave off the attack on Al Hudaydah, a city of 600,000 people and the gateway for most humanitarian aid to country. United Nations officials and others said they feared an assault on the city would worsen what is already the world’s worst humanitarian disaster.

More than 75 percent of Yemen’s population is dependent on food aid, and millions are on the brink of starvation.

Yemen’s civil war began in 2015 with a breakdown of political talks in the wake of Arab Spring protests that toppled the country’s longtime president, Ali Abdullah Saleh. Longstanding political divisions over power sharing gave way to military conflict and a variety of short-term alliances.

The Houthi rebels ousted the Yemeni government three years ago,…