The Saudi-led coalition geared up on Tuesday for an assault on Yemen’s main port, preparing to launch by far the biggest battle of a three-year-old war between an alliance of Arab states and the Houthi movement that controls Yemen’s capital.

The United Arab Emirates, one of the main members of the Western-backed alliance, has set a Tuesday deadline for the Iran-aligned Houthis to withdraw from the port of Hodeidah under UN-led negotiations or face an assault.

It would be the first time since they joined the war on behalf of Yemen’s exiled government that the foreign armies have attempted to capture such a well-defended major city.

Hodeidah, Yemen’s biggest port and the only port controlled by the Houthis, serves as the lifeline for the majority of Yemen’s population, which lives in Houthi-ruled territory.

READ: Saudi-led coalition strikes MSF hospital in Yemen

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.

It estimates 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 millions from aid and supplies.

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.

Local military sources said hundreds of Yemeni fighters as well as tanks and military supplies from the UAE arrived on Monday to reinforce troops, including Emiratis and Sudanese, in al-Durayhmi, a rural area 10 km (6.21 miles) south of Hodeidah.

UN: Assault on Yemen’s Hudaydah port could cost 250,000 lives

The sources said Yemeni forces allied to the Saudi-led coalition — drawn from southern separatists, local units from the Red Sea coastal plain and a battalion led by a nephew of late former president Ali Abdullah Saleh — had advanced and were “at the doors” of Hodeidah airport.

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, with roots in a Zaidi Shi’ite minority that ruled a thousand-year kingdom in Yemen until 1962, deny they are pawns of Iran. They say they have led a popular revolution against corruption and are defending the country from invaders.

“I think the Emiratis have done a good job in presenting compelling arguments about why an operation (on Hodeidah) could, in the end, tip the balance and apply enough pressure to bring the Houthis to the table,” a Western diplomat said on Monday.

“The Emiratis’ preparedness is crucial in this. This is possibly what we’re most concerned about.”

Grace period

European donor governments warned aid groups in Yemen on Saturday that “a military assault now looks imminent,” according to a message seen by Reuters. It said: “The Emiratis have informed us today that they will now give a 3-day grace period for the UN (and their partners) to leave the city.”

The UAE foreign ministry and government communication office did not immediately respond to Reuters requests for comment.

Yemen: Oxfam warns of famine disaster

The United Nations relocated foreign staff members from Hodeidah on Monday. The International Committee of the Red Cross said last week it had pulled 71 foreign staff out of Yemen.

The renewed push on Hodeidah comes amid increased tensions between Saudi Arabia and arch-foe Iran after the United States withdrew last month from an international nuclear agreement with Tehran, a move hailed by Riyadh and Abu Dhabi.

Saudi Arabia’s Western allies, under increasing scrutiny for selling arms to the states fighting in Yemen, have not publicly made clear whether they approve of an assault on Hodeidah.

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.

Houthi leader Abdul Malik al-Houthi has warned the group will attack oil tankers in the event of an assault on Hodeidah.

The coalition says one of the main justifications for its intervention is to protect Red Sea shipping, which brings Middle East oil and Asian goods to Europe through the Suez Canal.