A column of Yemeni government forces and vehicles take a position on the outskirts of the western port city of Hodeidah, Yemen, on June 14. According to reports, Yemeni government forces backed by the Saudi-led coalition continued to attack Houthi positions in Hodeidah. (Najeeb Almahboobi/EPA/Shutterstock)

The United States on Wednesday turned down a request from the United Arab Emirates to provide intelligence, overhead reconnaissance and mine-sweeping aid to a newly launched military offensive against the rebel-occupied Yemeni port of Hodeida, according to Emirati and U.S. officials.

The denial came as the United Nations continued a last-minute effort to avoid the attempt to retake the port, through which the majority of food, fuel and medical assistance flows to civilians in what aid groups already consider the world’s worst humanitarian disaster.

That effort, about which special U.N. envoy Martin Griffiths briefed the U.N. Security Council on Thursday, appeared to make little or no progress as the offensive by Yemeni forces backed by UAE troops began to move against the city of several hundred thousand. Griffiths has appealed to the rebels to evacuate the city.

The Trump administration is under pressure from lawmakers to limit U.S. support for the three-year-old war being conducted by a regional coalition led by Saudi Arabia and the Emiratis against the Iranian-backed Houthi rebel forces.

Most of that concern stems from the high civilian casualty rates caused by Saudi airstrikes throughout the war. The U.S. military, which considers both the Saudis and Emiratis close allies in the region, has provided intelligence and aerial refueling for the coalition, which administration officials said is continuing.

But it declined the request for additional help to assist the Hodeida operation.

A senior UAE official, who spoke on the condition of anonymity in deference to military briefers on the situation, said that France has agreed to assist in clearing the port with a minesweeper ship already in the region. French officials declined to comment.

The impetus for the Hodeida operation came primarily from the Emiratis, who have trained, equipped and assisted Yemeni ground forces loyal to internationally recognized Yemeni President Abdrabbuh Mansur Hadi.

Hadi’s government was ousted by the Houthis in the months after they seized Sanaa, the capital, in 2014. The Houthis joined forces with troops loyal to Ali Abdullah Saleh, Hadi’s predecessor, who ruled Yemen for three decades before he was deposed during the Arab Spring protests. The Houthis assassinated Saleh after he split with them late last year.

Over the past several months, UAE-backed forces have gradually retaken ground from the Houthis in southern Yemen and moved up the Red Sea coast toward Hodeida, which has been under rebel control since late 2015.

Griffiths, who has been struggling to implement a plan for political negotiations to end the war, tried this week to stop the effort to invade Hodeida, saying it would sharply worsen the humanitarian situation by making the transit of international assistance through the port even more difficult. He asked for a 72-hour delay of the invasion, but the UAE agreed to only 48 hours, a period that ended early Wednesday.

Although Saudi troops are said to be moving toward the city from the north, the major effort currently is from the UAE-supported force entering from the south. An attempt to retake the port by UAE shipborne forces in the Red Sea has been delayed as it became apparent that the Houthis were mining the waterway.

According to coalition plans, one goal of the operation is to avoid using airstrikes for anything except close support for advancing ground troops.

The coalition considers Hodeida the main conduit for Houthi arms supplies, which they maintain come primarily from Iran, and financial support in the form of $30 million to $40 million a month in customs fees and corruption.

The Emiratis say that rather than impeding the flow of aid, they are trying to facilitate it. “We are doing everything we can to continue and sustain humanitarian assistance, and in many cases improve it,” Yousef al-Otaiba, the UAE ambassador to the United States, said Thursday.

Responding to a chorus of alarm from relief agencies, a coalition statement on Wednesday announced what they called a “comprehensive five-point relief plan” to “safeguard and intensify the flow of humanitarian aid into the port of Hodeida.”

The plan appeared to assume the speedy capture by the coalition of the port and Hodeida city.

Dedicated shipping lanes from Saudi Arabia and the UAE would ferry food, medicine and fuel to the port and “teams on the ground” would distribute the supplies, the statement said. Economic support would “preserve trade and business” in Hodeida, it said. Electrical stations would remain operational.

The plan did not appear to address the concern that the battle for the city was likely to interrupt the flow of aid, which relief workers say would be catastrophic for Yemen in its fragile state, with a third of the population, or 8 million people, on the brink of famine. Aid agencies say there is no substitute for the Hodeida port and have urged the coalition to halt the offensive.

Fighting Thursday was concentrated south of Hodeida and had not yet reached the city center, where civilians moved freely, the International Rescue Committee said in a briefing note. Some civilians appeared to be fleeing Hodeida, despite reports that the rebels were preventing people from leaving, the group said.

The Emirates News Agency said Thursday night that UAE troops and allied forces had reached the outskirts of Hodeida airport after a “rapid advance” and that 96 rebel fighters had been killed by coalition airstrikes. The claims could not be independently confirmed.

Mohamed Ibrahim, a schoolteacher who lives near the airport and was reached on Thursday night, said he did not hear sounds of fighting and there were no indications that rebel troops in the area were retreating.

The UAE had previously announced that four of its soldiers were killed after the offensive began early Wednesday. The senior UAE official said they were on a landing craft about 20 miles offshore in the Red Sea, which was hit by a Houthi-launched missile.

Fahim reported from Istanbul. Ali Al-Mujahed contributed from Sanaa, Yemen.