The battle for Hodeidah is expected to be by far the biggest of a three-year war between an alliance of Arab states and the Iranian-backed Houthis.
Before dawn Wednesday, convoys of vehicles appeared to be heading toward the rebel-held city, according to videos posted on social media.
Before the war, over 70 percent of Yemen's food and fuel imports came through Hudaida, accounting for over 40 percent of the nation's customs income.
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, of which 8.4 million fee pre-famine conditions.
Yemen's exiled government "has exhausted all peaceful and political means to remove the Houthi militia from the port of Hodeida", it said in a statement.
"The coalition's operations to liberate Hodeidah is part of the Coalition's unwavering commitment to support the people of Yemen against the tyranny imposed by Iranian-backed militias that are spreading chaos and destruction in Yemen", Khalid bin Salman wrote in a tweet.
Local military sources said hundreds of Yemeni fighters as well as tanks and gear from the UAE arrived on Monday to reinforce troops, including Emiratis and Sudanese, in al-Durayhmi, a rural area 10 km (6.21 miles) south of Hodeidah. In August 2015, air strikes disabled four giant mobile cranes, drastically slowing the unloading of food until they were replaced by the U.S. - which supports the coalition - this January.
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The UAE, which is involved in the ground assault, had given the Houthis a deadline of Wednesday morning to withdraw from Hodeidah or face attack.
The United Nations on Monday withdrew its global staff from Al Hudaydah, saying an attack would "impact hundreds of thousands of innocent civilians".
Hudaydah is a lifeline for people living in rebel-held areas, serving as the most important point of entry for the basic supplies needed to prevent starvation and a recurrence of a cholera epidemic that affected a million people previous year.
Coalition sources said the alliance carried out 18 air strikes on Houthi positions on the outskirts of Al Hudaydah on Wednesday.
Riyadh says the Houthis use the port to smuggle Iranian-made weapons, including missiles that have targeted Saudi cities - accusations denied by the group and Iran.
"In a prolonged worst case, we fear that as many as 250,000 people may lose everything - even their lives".
The U.N. special envoy to Yemen, Martin Griffiths, said the world body was talking to both sides to try to avert a battle.
"Under worldwide 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, U.N. humanitarian coordinator for Yemen. In a report on Tuesday, the Wall Street Journal confirmed that the U.S.is providing the Saudis with "intelligence to fine-tune their list of airstrike targets" in Hodeida.
At the same time, the Trump administration has been seeking ways to demonstrate a tough stance toward Tehran's support of armed groups across the Middle East. Yemen, where Iran has provided only limited support for the Houthi movement, is seen as an easier starting point for that campaign than countries where its proxy support is more established.