The Saudi-led coalition did not immediately acknowledge the incident. The U.S. Navy's 5th Fleet, whose area of responsibility includes the Red Sea, referred questions to the Pentagon, which did not immediately respond to a request for comment.
Forces loyal to Yemen's exiled government and fighters led by Emirati troops had neared Hodeida in recent days. The port is some 150 kilometers (90 miles) southwest of Sanaa, Yemen's capital, which has been in Houthi hands since September 2014. The Saudi-led coalition entered the war in March 2015.
The United Nations and other aid groups already had pulled their international staff from Hodeida ahead of the assault.
The port remains open, with supplies arriving. Several ships have arrived in recent days, including oil tankers, and there has been no word from the coalition or U.N. to stop work, according to a senior port official, who spoke on condition of anonymity because he was not authorized to talk to the press.
Aid groups nevertheless warned of disaster.
Robert Mardini, the regional director for the Red Cross, said the push on Hodeida "is likely to exacerbate an already catastrophic humanitarian situation in Yemen. The population has already been weakened to extreme levels," he added.
David Miliband, the head of the International Rescue Committee, called the offensive "an attack on the political and diplomatic process to bring peace to Yemen." He said the U.N. Security Council must act to secure a cease-fire before the people of Hodeida "suffer the same fate as those in Aleppo, Mosul or Raqqa."
Over 10,000 people have been killed in Yemen's civil war, which has displaced 2 million more and helped spawn a cholera epidemic. Saudi-led airstrikes that killed large numbers of civilians and damaged vital infrastructure. Meanwhile, the U.N. and Western nations say Iran has supplied the Houthis with weapons from assault rifles up to the ballistic missiles they have fired deep into Saudi Arabia, including at the capital, Riyadh.
The coalition has blocked most ports, letting supplies into Hodeida in coordination with the U.N. The air campaign and fighting have disrupted other supply lines, causing an economic crisis that makes food too expensive for many to afford.
The U.N. says some 600,000 people live in and around Hodeida, and "as many as 250,000 people may lose everything — even their lives" in the assault. Already, Yemeni security officials said some were fleeing the fighting.
"We hear sounds of explosions. We are concerned about missiles and shells. Some workers have left to their villages for fear of the war," said Mohammed, a Hodeida resident who gave only his first name for fear of reprisals.
Aid workers had similar worries.
"We have had more than 30 airstrikes within 30 minutes this morning around the city. Some civilians are entrapped, others forced from their homes," said Jolien Veldwijk, the acting country director of the aid group CARE International, which works in Hodeida. "We thought it could not get any worse, but unfortunately we were wrong."
The new U.N. envoy to Yemen, Martin Griffiths, tweeted that he was "extremely concerned" by the violence, calling on all parties to exercise restraint. Griffiths' recent appointment as envoy and his push for new negotiations may have encouraged the Saudi-led coalition to strengthen its hand ahead of any peace talks with the Houthis.
The attack comes as Washington has been focused on President Donald Trump's recent summit with North Korean leader Kim Jong Un. The U.S. has been offering targeting information to the Saudi-led coalition, as well as refueling their warplanes, though its role in Wednesday's assault wasn't immediately clear.
Associated Press writers Ahmed al-Haj in Sanaa, Yemen; Maggie Michael in Aden, Yemen; and Samy Magdy in Cairo contributed to this report.