A Saudi-led alliance of Arab states has launched an attack on Yemen's main port city, the largest battle of the war.

The move is aimed at bringing 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".

Ground battles raged near Hodeidah airport and al-Durayhmi, a rural area 10km south of the city, media controlled by the Arab states and their Yemeni allies reported.

The assault marks the first time the Arab states have tried to capture such a heavily-defended major city since joining the war three years ago against the Iran-aligned Houthis, who control the capital Sanaa and most of the populated areas.

The port is the main route for food to reach most Yemenis, 8.4m of whom are already on the verge of famine.

The Houthis deployed military vehicles and troops in the city centre and near the port, as warplanes struck the coast to the south, a resident said.

People fled by routes to the north and west.

CARE International, one of the few aid agencies still there, said 30 air strikes hit the city within half an hour.

"Some civilians are entrapped, others forced from their homes. We thought it could not get any worse, but unfortunately we were wrong," said CARE's acting country director, Jolien Veldwijk.

Saudi-owned Al Arabiya TV quoted witnesses describing "concentrated and intense" bombing near the port itself.

"Under international humanitarian law, parties to the conflict have to do everything possible to protect civilians and ensure they have access to the assistance they need to survive," said Lise Grande, United Nations humanitarian coordinator for Yemen.

United Nations calls for restriant

The UN special envoy to Yemen, Martin Griffiths, said the world body is talking to both sides to try to avert a battle.

"We call on them to exercise restraint and engage with political efforts to spare Hodeidah a military confrontation," he tweeted.

UN refugee chief Filippo Grandi said there was a danger Yemenis might flee across the sea to Somalia or Djibouti.

Port workers said that five ships were docked at Hodeidah port unloading goods, but no new entry permits would be issued today.

The Arab states say they will try to keep the port running and can ease the crisis once they seize it by lifting import restrictions they have imposed.

The operation began after a three-day deadline set by the United Arab Emirates for the Houthis to quit the port.

Western countries have quietly backed the Arab states diplomatically, while mostly avoiding direct public involvement in the conflict.

A major battle could test that support, especially if many civilians are killed or supplies disrupted.

The United States, Britain and France all sell billions of dollars of weapons a year to the Arab countries.