WASHINGTON (Reuters) - The United States has warned its close ally the United Arab Emirates not to launch an assault on Yemen’s port city of Hodeidah, which United Nations experts fear could precipitate a new humanitarian crisis, U.S. officials said on Tuesday.

FILE PHOTO: A man walks during a visit by a U.N. delegation to the Red Sea port of Hodeidah, Yemen July 26, 2017. REUTERS/Abduljabbar Zeyad

The warning was issued as Emirati-backed Yemeni forces moved within 6 miles (10 km) of the Red Sea port, a lifeline for humanitarian supplies for the country’s war-wracked populace. The port is controlled by Yemen’s Houthi movement.

U.N. officials have warned that an assault on Hodeidah, which has a population of about 600,000, would cause a humanitarian calamity. A U.N. contingency plan projects that in the worst-case scenario of a prolonged siege, tens of thousands could die.

“In terms of an operation on Hodeidah, we’ve been quite clear that we want to engage on the political track,” a Western official said on condition of anonymity, adding that U.N. envoy for Yemen Martin Griffiths “now has the best chance of anyone in quite some time of getting it moving.”

“This is a point that we’ve made at the top level,” the official added. “The Saudis and Emiratis have made clear that they would not move without a joint understanding from their friends and partners of what the next steps are - what the day after will be - if there were an assault on Hodeidah.”

U.S. officials met at the White House on Tuesday to discuss the crisis, two sources with knowledge of the meeting said. It was unclear whether any decisions were reached.

A spokesperson for the White House National Security Council said Washington 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 authorized to speak publicly about the matter and asked not to be named.

“We expect all parties to abide by the Law of Armed Conflict and avoid targeting civilians or commercial infrastructure,” the spokesperson said.

The United States and the Emirates are close economic and security allies, working together against militant groups in Yemen and elsewhere.

Any direct American support to an Emirati ground offensive against Hodeidah would mark a major departure from U.S. policy toward Yemen, which has sought to focus U.S. military activity against al Qaeda-affiliated militants.

The United States also has repeatedly urged the Saudi-led coalition, both under President Donald Trump and his predecessor, Barack Obama, to prevent civilian casualties.

Critics of Washington have said, however, that its refueling of Saudi-led coalition jets, arms sales and limited U.S. intelligence support make it complicit in civilian casualties caused by the coalition.

Yemeni political sources said on Monday that U.N. envoy Griffiths was in the capital, Sanaa, to broker a deal to avert a possible assault on Hodeidah, under which the United Nations would take control of the port.