The current configuration of forces pits the predominantly Shiite Houthi rebels, who were long marginalized by Saudi-backed proxy groups in Yemen, against Sunni tribal and other militia groups that are backed by the Emiratis and Saudis.
Diplomats involved in behind-the-scenes negotiations say that the United Arab Emirates officially warned the British government on Friday that an attack on Al Hudaydah was imminent. The Emiratis said they would give three days for humanitarian workers and nongovernmental organizations to flee the city.
The International Committee for the Red Cross removed its staff from the city over the weekend.
The United Nations worked out terms with the Houthi rebels on Sunday and planned to have its foreign staff evacuate from the city in multiple convoys on Monday. United Nations agencies planned to leave in place a skeleton crew of Yemenis from Al Hudaydah to try to keep their humanitarian mission going.
Diplomats familiar with the situation say they believe that the Emiratis, who are leading the push for an attack, are looking to launch their planned assault while Washington’s attention is focused this week on the summit meeting between President Trump and the North Korean leader Kim Jong-un.
The United States has backed the Arab states in the war, but Abu Dhabi has received powerful pushback from various American officials who see the idea of an urban assault on a densely populated city as an unmitigated disaster, in both military and humanitarian terms.
Secretary of State Mike Pompeo said in a statement that he had spoken to Emirati leaders to emphasize the American wish to keep humanitarian supply lines open and to preserve a political process between the opposing sides in Yemen.
Mr. Pompeo said that in his conversation with the Emiratis he had made clear the United States’ “desire to address their security concerns while preserving the free flow of humanitarian aid and lifesaving commercial imports,” the statement said.