The United Nations and other aid groups already had pulled their global staff from Hodeida ahead of the rumored assault. However, the Houthi rebels have repeatedly refused a deal to hand over control of the Al Hodeidah port to UN-monitored worldwide forces. An assault was "likely to exacerbate an already catastrophic humanitarian situation", Red Cross spokeswoman Marie-Claire Feghali said.
A view of the Red Sea port city of Hodeidah, Yemen May 10, 2017.
The United Nations said it was engaged in "intense" shuttle diplomacy between the Houthis and coalition leaders Saudi Arabia and the UAE to avert the attack.
Al Arabiya television also reported that an offensive had been launched, but there was no immediate word from the state media in either Saudi Arabia or the UAE.
"The liberation of Hodeidah port is a turning point in our struggle to recapture Yemen from the militias that hijacked it to serve foreign agendas", the exiled government said in a statement carried by state-run Yemeni media.
Al Hodeidah, the second largest port in the country, is the main conduit for humanitarian supplies into a country teetering on the brink of starvation.
The UAE, one of the main members of the coalition backing the government of Abd Rabbuh Mansour Hadi, had set a Tuesday deadline for the Houthi rebels to withdraw from the city under UN-led negotiations or face an assault.
Emirati-led troops have advanced along the southwestern coast to the outskirts of Hodeidah under a coalition strategy to box in the Houthis in the capital Sanaa and choke off their supply lines to force them to the negotiating table.
More than 10,000 people have been killed in Yemen's civil war, while the United Nations says Iran has supplied the Houthis with weapons from assault rifles up to the ballistic missiles.
The Arab coalition has repeatedly said that it will protect Al Hodeidah's lifeline and seek to secure the lives of civilians.
It estimates that 600,000 people live in the area, and in a worst-case scenario, a battle could cost up to 250,000 lives, as well as cutting off aid and other supplies to millions of people facing starvation and disease.
Washington had also cautioned against the assault.
The US position on Hodeidah continues to shift wildly, however. It said: "The Emiratis have informed us today that they will now give a three-day grace period for the United Nations (and their partners) to leave the city".
It wasn't immediately clear what specific American support the coalition was receiving Wednesday.
Coalition warplanes and warships were carrying out strikes on Houthi fortifications to support ground operations by Yemeni troops massed south of the Red Sea port, the internationally recognised Yemeni government in exile said in a statement.
The war pits the Houthis against the Western-backed Sunni Muslim states, which intervened in 2015 to restore the exiled government and thwart what Riyadh and Abu Dhabi see as expansionist aims of their Shi'ite Muslim foe Iran.
The Houthis deny they are Iranian pawns and say their revolt aims to target corruption and defend Yemen from invaders. The United Arab Emirates has warned the Ansarullah movement to withdraw from the port city.
Reem al-Hashimy, the UAE minister of state for worldwide cooperation, has said if the port is wrested from the Houthis, the coalition could ease controls, aimed at denying the group arms, and so improve the flow of goods and aid into Yemen. The Houthis have been using the port to smuggle weapons into the country. The accusations are denied by the group and Iran.
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