UNITED NATIONS/WASHINGTON - The United Nations is engaged in "intense" shuttle diplomacy between the Iran-aligned Houthi group in Yemen, the United Arab Emirates and Saudi Arabia in a bid to avert an attack on Yemen's Hodeidah port city, U.N. chief Antonio Guterres said on Monday.A Saudi-led coalition military attack or siege on the Houthi-held western city, long a target in the war, could cost up to 250,000 lives in a worst-case scenario, the United Nations has warned. The Red Sea port is a lifeline for millions of people, handling most of the country's commercial imports and humanitarian aid supplies.The U.N. Security Council met behind closed doors on Monday at the request of Britain to be briefed on the situation after heavy fighting erupted near Hodeidah on Friday and Saturday."We are, at the present moment, in intense consultation," Guterres told reporters on Monday. “I hope that it will be possible to avoid a battle for Hodeidah.”Correspondence sent from European donor governments to NGOs in Yemen on Saturday warned that "a military assault now looks imminent," according to the text of the correspondence seen by Reuters."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 United Nations relocated members of its international staff from Hodeidah on Monday.Guterres said U.N. Yemen envoy Martin Griffiths was shuttling between Sanaa and also the UAE and Saudi Arabia.U.S. Secretary of State Mike Pompeo on Monday called on all parties "to honor their commitments to work with the U.N."He said in a statement that he had spoken with leaders of the United Arab Emirates and "made clear our desire to address their security concerns while preserving the free flow of humanitarian aid and life-saving commercial imports."Pompeo's statement, while cautionary, did not explicitly urge the Emiratis to avoid an assault on Hodeidah.Oxfam, one of the world's biggest disaster relief charities, warned about a potential assault."Our message is clear ... if this assault happens then it would be very difficult to imagine how difficult the situation the people in Yemen, particularly in Hodeidah, will be in," said Muhsin Siddiqui, country director of Oxfam.Griffiths has been working on a peace plan that calls on the Houthi group to give up its ballistic missiles in return for an end to a Saudi-led coalition bombing campaign and a transitional governance agreement. He is due to brief the U.N. Security Council later this month.Previous U.N. efforts have failed to end the more than three-year-old conflict which pits the Houthis, who seized the capital Sanaa in 2014, against other Yemeni forces backed by a coalition loyal to exiled President Abd-Rabbu Mansour Hadi and led by U.S. allies Saudi Arabia and the United Arab Emirates.After briefing the Security Council on Monday, U.N. aid chief Mark Lowcock told reporters that "if for any period Hodeidah were not to operate effectively the consequences in humanitarian terms would be catastrophic."