A Saudi-led alliance of Arab states launched the largest assault of Yemen's war on Wednesday with an attack on the country's main port, aiming to bring the ruling Houthi movement to its knees at the risk of worsening the world's biggest humanitarian crisis.

Arab warplanes and warships pounded Houthi fortifications to support ground operations by foreign and Yemeni troops massed south of the port of Hodeidah in operation "Golden Victory".

The United Nations fears the assault could drastically worsen already desperate conditions in the region's poorest country. The city and surrounding area are home to 600,000 people, and the port is the main route for food and aid to reach most Yemenis, 8.4 million of whom are on the verge of famine.

The Arab states hope for a swift victory that would force the Iran-aligned Houthis to negotiate. They say they will try to keep the port running and can ease the crisis by lifting importrestrictions if they seize it.

Western countries, particularly the United States and Britain, have quietly backed the Arab states diplomatically and sell them billions of dollars a year in arms, but have mostly avoided direct public involvement so far in the Yemen conflict.

Exacerbate catastrophe

The United Nations had been trying to get the parties to reach a deal to avert an attack. An assault was "likely to exacerbate an already catastrophic humanitarian situation," Red Cross spokeswoman Marie-Claire Feghali said.

The alliance intervened in Yemen to restore the government of President Abd-Rabbu Mansour Hadi, who was driven from the capital Sanaa and into exile in 2014.

Riyadh and Abu Dhabi see the Houthi rise as expansionism by their Shi'ite foe, Iran. The Houthis, drawn from a Shi'ite minority that ruled a thousand-year kingdom in Yemen until 1962, deny they are Iranian pawns. They say they took power through a popular revolt against corruption and are now defending Yemen from an invasion by its Western-backed neighbours.

Yemen lies beside the southern mouth of the Red Sea, one of the world's most important trade routes, where oil tankers pass from the Middle East through the Suez Canal to Europe.

The UAE has said coalition forces plan to keep the port operational but warned that the Houthis could sabotage infrastructure and place land and sea mines as they withdraw.