The battle for Hodeidah is expected to be by far the biggest of a three-year war..
The Houthi official responsible for foreign affairs, Hisham Sharaf, said the advance was a military escalation aimed at hampering United Nations efforts, Houthi-run media said. Houthi forces have fired such missiles at ships previously.
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.
A source from the Yemeni National Resistance said that a large number of forces from Brigades AlAmalaqah and the Tihama Resistance Forces, armed with modern weapons and in high morale, have arrived at the outskirts of Al Hodeidah city to continue their build-up in the front lines in preparation for the battle of rolling back the Houthi militias and putting and end to the coup plot in Yemen.
The United Nations fears the assault could drastically worsen already desperate conditions in the region's poorest country.
Saudi Arabia's Western allies, which sell arms to coalition states, have not said publicly if they approve of the assault.More news: Raptors hiring Nick Nurse as head coach
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.
Forces loyal to Yemen's exiled government and irregular fighters led by Emirati troops had neared Hudaida in recent days.
For a little more than three years, Yemen has been locked in a seemingly intractable civil war that has killed almost 10,000 people and pushed millions to the brink of starvation. The support has continued despite worldwide alarm over the coalition's air campaign against the Houthis, which has killed thousands of civilians in airstrikes that human rights groups have alleged are frequently indiscriminate.
The U.N.'s humanitarian coordinator for Yemen, Lise Grande, warned that a prolonged siege of the port could lead to the deaths of up to a quarter-million Yemenis due to starvation and disease.
The UN said 600,000 people live in and around Hodeidah, and "as many as 250,000 people may lose everything - even their lives" in an assault.
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.More news: WH adviser Navarro apologizes for 'special place in hell' slam at Trudeau
In Geneva, ICRC spokeswoman Marie-Claire Feghali said that the assault was "likely to exacerbate an already catastrophic humanitarian situation in Yemen", where water and electricity networks are vital to the civilian population's survival.
The Sunni Muslim Gulf states intervened in Yemen to restore the exiled government and thwart what Riyadh and Abu Dhabi see as expansionist objectives of their Shi'ite foe Iran.
The Houthis deny they are Iranian pawns and say their revolt aims to target corruption and defend Yemen from invaders.
The Houthi militia has repeatedly fired missiles at Saudi Arabia, which the United States and UN experts say are of Iranian origin, a claim Tehran denies.
Speaking to reporters this week, Mattis said that US support would continue at its current levels, and suggested USA aerial refueling, which lengthens flight times, increased the accuracy of Saudi and UAE bombing raids and so cut down on civilian casualties.
Yemen lies beside the southern mouth of the Red Sea, one of the most important trade routes in the world for oil tankers, which pass near Yemen's shores while heading from the Middle East through the Suez Canal to Europe.More news: Murder probe as hunt under way for killers of Polish man