NEW YORK (Reuters) - The United Nations is concerned that a U.S. plan to blacklist Yemen’s Houthi movement on Tuesday will hinder its efforts to assess a decaying oil tanker that is threatening to spill 1.1 million barrels of crude oil off the war-torn country’s coast. The tanker Safer has been stranded off Yemen’s Red Sea oil terminal of Ras Issa for more than five years, and U.N. officials have warned it could spill four times as much oil as the 1989 Exxon Valdez disaster off Alaska. Houthi authorities gave long-awaited approval in November for a visit to assess the tanker. A U.N. team, which includes a private company contracted by the world body to do the work, aims to travel to the tanker early next month. “We are continuing to prepare for the Safer tanker assessment and are broadly on track with this work,” U.N. spokesman Stephane Dujarric said on Friday. “However we have to look at potential impact of the (U.S. designation) on the Safer mission – there are questions about potential legal jeopardy for the people who are going to do the mission, which of course we all want to avoid,” he said. U.S. Secretary of State Mike Pompeo announced the move against the Iran-aligned Houthis on Sunday. It will come into effect next Tuesday, the last full day in office of Republican President Donald Trump’s administration. Democratic President-elect Joe Biden takes office next Wednesday. The designation could be revoked by Biden’s administration. The Houthis control the area where the tanker is moored and the national oil firm that owns it. The designation freezes any U.S.-related assets of the Houthis, bans Americans from doing business with them and makes it a crime to provide support or resources to the movement. U.N. officials are worried the designation will scare private-sector companies away from any business related to Yemen. When asked about the U.N. concerns about the tanker mission, a State Department spokesperson said: “We have expressed our readiness to work with relevant officials at the United Nations, with international and nongovernmental organizations, and other international donors to address sanction implications.” Washington has said it will issue licenses and exemptions to allow the United Nations and aid groups to continue working in Yemen, which the United Nations describes as the world’s largest humanitarian crisis, with 80% of the people in need. But top U.N. officials have said little is known about possible exemptions and warned that the U.S. move would push Yemen, which relies almost solely on imports, into a large-scale famine and chill peace efforts. A Saudi Arabia-led military coalition intervened in Yemen in 2015, backing government forces fighting the Houthis. U.N. officials are trying to revive peace talks to end the war as the country’s suffering is also worsened by an economic crisis, currency collapse and the COVID-19 pandemic.