The operation has started after the passing of a three-day deadline set by the United Arab Emirates for the Houthis to quit the port of Hodeidah
A Saudi-led alliance of Arab states launched the largest assault of Yemen's war on Wednesday with an attack on the main port city, aiming to drive 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 assault marks the first time the Arab states have tried to capture such a heavily-defended major city since they joined the war three years ago against the Iran-aligned Houthis, who control the capital Sanaa and most of the populated areas.
The Houthis had deployed military vehicles and troops in the city centre and near the port, as coalition warplanes flew overhead striking a coastal strip to the south, one resident, speaking on condition of anonymity, told Reuters. People were fleeing by routes out to the north and west.
CARE International, one of the few aid organizations still operating in Hodeidah, said 30 air strikes had hit the city within half an hour on Wednesday morning.
"Some civilians are entrapped, others forced from their homes. We thought it could not get any worse, but unfortunately we were wrong," said CARE acting country director Jolien Veldwijk.
Saudi-owned Al Arabiya TV quoted witnesses describing "concentrated and intense" bombing near the port itself.
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.
"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. Right now, nothing is more important," said Lise Grande, UN humanitarian coordinator for Yemen, who is in Sanaa.
UN refugee chief Filippo Grandi said there was a danger of a more immediate crisis if Yemenis began to abandon their homes in large numbers.
Port workers told Reuters five ships were docked at Hodeidah port unloading goods, but no new entry permits would be issued on Wednesday due to the fighting.
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.
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. A major battle could test that support, especially if many civilians are killed or supplies disrupted.
The operation began after the passing of a three-day deadline set by the United Arab Emirates, one of the coalition's leaders, for the Houthis to quit the port.
"The liberation of the port is the start of the fall of the Houthi militia and will secure marine shipping in the Bab al-Mandab strait and cut off the hands of Iran, which has long drowned Yemen in weapons that shed precious Yemeni blood," the Arab-backed government-in-exile said in a statement.
Houthi leader Mohammed Ali Al-Houthi, who has threatened attacks on oil tankers, warned the alliance not to attack the port and said on Twitter his forces had struck a coalition barge. There was no immediate confirmation from the coalition.
The Arab states' aim is to box in the Houthis in Sanaa, cut their supply lines and force them to the negotiating table.
A Yemeni anti-Houthi military official said the alliance had brought to bear a 21,000-strong force. It includes Emirati and Sudanese troops as well as Yemenis, drawn from southern separatists, local Red Sea coast fighters and a battalion led by a nephew of late ex-president Ali Abdullah Saleh.
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.
With its military intervention in Yemen, the alliance aims to restore the government of President Abd-Rabbu Mansour Hadi, who was driven from Sanaa and into exile in 2014.
Riyadh and Abu Dhabi see the Houthi rise as expansionism by their Shia foe, Iran. The Houthis, drawn from a Shia minority that ruled a thousand-year kingdom in Yemen until 1962, say they took power through a popular revolt against corruption and are now defending Yemen from invasion by its neighbours.
Yemen has been in crisis since 2011 mass protests that ended the 33-year rule of Saleh. A Saudi-brokered transition brought Hadi to power in a government that sidelined the Houthis, who become disgruntled and captured much of the country.
For a time Saleh joined forces with the Houthis, although they turned on each other last year and Saleh was killed. Parts of Yemen are also held by al Qaeda and Islamic State militants.
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.