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. Some 70 percent of Yemen's food enters the country via the port, as well as the bulk of humanitarian aid and fuel supplies.
Before dawn on Wednesday, convoys of vehicles appeared to be heading towards the rebel-held city, according to videos posted on social media.
Bombardment was heavy, with one aid official reporting 30 strikes in 30 minutes. "We thought it could not get any worse, but unfortunately we were wrong".
Emirati forces with Yemeni troops moved in from the south near Hudaida's airport, while others sought to cut off Houthi supply lines to the east, the officials said. They spoke on condition of anonymity as they weren't authorized to brief journalists.
Yemen's biggest port, Hodeidah is the lifeline for the majority of Yemen's population, which lives in Houthi territory.
The UAE, a pillar of the anti-Houthi coalition, says retaking Al Hudaydah is necessary to force the rebels to make concessions.
The Saudi-led coalition did not immediately acknowledge the incident.
With anti-government forces embedded in the city, military victory in Hodeida "seems a very tall order" and an assault on the city would only lead to further bloodshed, he warned.
Hudaydah, 140km (90 miles) west of the capital Sanaa, was Yemen's fourth-largest city and a major economic hub before rebels took control of it in late 2014.
Speaking to reporters this week, Mattis said that USA support would continue at its current levels, and suggested US aerial refueling, which lengthens flight times, increased the accuracy of Saudi and UAE bombing raids and so cut down on civilian casualties.
The United Nations and other aid groups already had pulled their global staff from Hodeida ahead of the rumored assault.
And just to the north is the Ras Isa oil terminal - which served the Marib oilfields and was the country's main export terminal - and the nearby port of Saleef. 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.
Aid groups nevertheless warned of disaster.
"All member states, led by the US, must demand that vital humanitarian and commercial imports through Hodeidah are maintained", said David Miliband, head of the International Rescue Committee.
Defense Secretary Jim Mattis on Monday acknowledged the U.S. continues to provide support to the Saudi-led coalition.
Hudaydah became even more important after the conflict between the Houthis and the government escalated in March 2015, when the Saudi-led coalition intervened.
Riyadh says the Houthis use the port to smuggle Iranian-made weapons, including missiles that have been launched at Saudi cities - accusations denied by the group and Tehran.
"If there is a viable transition to a legitimate presence in Hodeidah this will alleviate the suffering of the Yemeni people and facilitate the flow of goods and aid into Yemen", Hashimy said in a briefing.
"We hear sounds of explosions. This is possibly what we're most concerned about".
The new United Nations envoy to Yemen, Martin Griffiths, tweeted that he was "extremely concerned" by the violence, calling on all parties to exercise restraint.
The Trump Administration, which provides military support to the coalition, had asked the Emirates to hold off on beginning an operation until after United Nations envoy Martin Griffiths presented a new plan for jump-starting peace talks.
Washington had also cautioned against the assault.