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The UAE, one of the main members backing Yemen's exiled government of Abd Rabbuh Mansour Hadi, set a deadline on Tuesday for Iran-backed Houthi rebels to withdraw from the city or face an offensive.

Shia Houthi rebels have controlled the Red Sea port for years, crucial to food supplies in a nation on the brink of famine. The battle for Hodeida could signal the first major urban fighting for the Saudi-led coalition, one that would prove deadly for both combatants and civilians alike.

Videos circulated on social media showed convoys of vehicles heading towards the rebel-held city before dawn on Wednesday. The sound of heavy, sustained gunfire clearly could be heard in the background.

Saudi-owned satellite news channels and later state media announced the battle had begun, citing military sources. They also reported coalition airstrikes and shelling by naval ships.

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. "Liberation of the port of Hodeida is a milestone in our struggle to regain Yemen from the militias."

The Houthi-run Al Masirah satellite news channel later acknowledged the offensive, claiming rebel forces hit a Saudi coalition ship near Hodeida with two missiles. Houthi forces have fired missiles at ships previously.

"The targeted ship was carrying troops prepared for a landing on the coast of Hodeida," the channel said.

Forces loyal to Yemen's exiled government and irregular 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 held by the Houthis since they swept into the city in September 2014. The Saudi-led coalition entered the war in March 2015 and has received logistical support from the US.

Emirati Minister of State for Foreign Affairs Anwar Gargash earlier told French newspaper Le Figaro the deadline for a withdrawal from Hodeida by the Houthis expired early Wednesday morning.

Catastrophic consequences

The United Nations and other aid groups already had pulled their international staff from Hodeida ahead of the rumored assault.

The city is home to 600,000 people and is the entry point for 70 percent of Yemen's imports, including vital aid supplies for civilians in the conflict-wracked country.

Aid groups operating in Yemen have warned of catastrophic consequences from the assault, which the UN says puts 250,000 people in the line of fire.

The pro-government forces are a mix of local fighters, those loyal to President Abd Rabbo Mansour Hadi, and supporters of the ex-head of state, Ali Abdullah Saleh, who was killed in December by his former Houthi allies.

They are backed on the ground by the UAE, while Saudi Arabia has been leading a campaign of air strikes.

Hadi visited the UAE on Tuesday for his first official visit since a crisis in his relations with Abu Dhabi.

The UAE has been expanding its influence in southern Yemen, and also back separatists who wrenched control of the south of the country from Hadi in January.