Four Emirati soldiers were killed in the opening hours of the operation when their troop vessel was struck by an anti-ship missile about 20 miles outside the harbor, said the Emirati official, who spoke on condition of anonymity to discuss the operation.

Emirati officials have said the goal of the military offensive is to press the Houthi fighters into submission by denying them a port that by one Emirati estimate yields the rebels between $30 million to $40 million a month in port fees and other revenues. The goal, they said, was to force the Houthis to accept a political solution.

Fighting on Thursday appeared to be concentrated around the city’s airport, the first strategic target the Arab coalition is trying to seize before battling for control of the vital port facilities. Approximately 80 percent of the country’s humanitarian aid enters though the port.

Local forces trained and financed by the Emirati military clashed on the southern outskirts of the city with fighters loyal to the Yemeni rebel movement, the Houthis, that has controlled Al Hudaydah for the last three years.

Aid groups say there have been no reports of shelling or bombing inside the city. A Saudi military spokesman, Col. Turki al-Maliki, said the coalition plan was to take control of the airport, seaport and the route leading to the capital, Sana, but not to engage in urban warfare.

But in Yemen’s protracted civil war, which has killed approximately 10,000 people and led to tens of thousands more deaths from sickness and starvation, international aid agencies are wary of predictions by the Saudis and the Emiratis that they could snatch a quick victory in Al Hudaydah’s complex urban environment.

Al Hudaydah is a city of 600,000 people. About a quarter of a million people are in danger of injury or death in an urban assault, the United Nations said.