Saudi and UAE-backed forces have began an offensive on the Yemeni port city of Hodeidah early on Wednesday despite warnings from Western governments and international aid groups.
Saudi-owned Al Arabiya TV announced the attack, citing military sources.
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.
Hadi's government said in a statement reported by Saudi media on Wednesday that all political and peaceful approaches to drive the Houthis out of Hodeidah had been exhausted.
It added that recapturing the port city will be the beginning for "liberating" all of Yemen from rebels, including the capital Sanaa.
The embattled Yemeni president had visited the UAE in a rare move on Tuesday.
The United Nations – which has been evacuating staff this week – had warned that an attack on the densely populated city could cost up to 250,000 lives.
Washington had also cautioned against the assault.
Last week, a spokesperson for the White House National Security Council said the US opposed any effort by the Emirates and Yemeni troops it backs to seize the city.
"The United States has been clear and consistent that we will not support actions that destroy key infrastructure or that are likely to exacerbate the dire humanitarian situation that has expanded in this stalemated conflict," said the spokesperson, who was not authorised to speak publicly about the matter and asked not to be named.
More than 10,000 people have been killed since Saudi Arabia led a bombing campaign against the Houthis. The conflict has also caused a cholera outbreak and brought the already impoverished nation to the verge of famine.
Saudi Arabia views the Houthis as Iranian proxies, but the rebels deny receiving military support from Tehran.
On Monday, the UN Security Council voiced support for diplomatic efforts to avert the attack on Hodeidah.
UN aid chief Mark Lowcock, who also briefed the council, said an attack on Hodeidah would be "catastrophic" and that aid agencies were hoping to "stay and deliver" in Yemen, which the UN describes as the world's worst humanitarian crisis.
Yemen relies on imports for 90 percent of its food and 70 percent of those transit through Hodeidah, Lowcock said.
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Eleven humanitarian aid agencies, including Oxfam and Save the Children, separately wrote to British Foreign Secretary Boris Johnson earlier this week urging him to warn the coalition that it will lose British support if it attacks Hodeidah.
"If an attack does take place, casualties on all sides will be high with a likely catastrophic impact on the civilian population," they wrote.
UAE Minister of State for Foreign Affairs Anwar Gargash told the Houthis on Tuesday that the "final hour" is approaching in Hodeidah.
"If the current situation continues we will have a stalemate politically," he said. "The last thing we want is to prolong the war in Yemen."
Abu Dhabi's critics say the UAE military operations have focused on port cities to control commerce in the strategic region, not to drive out the Houthis.