The United Nations is "deeply concerned" that the Houthi group's delay in granting security guarantees for UN experts tasked with examining an oil tanker anchored off the coast of Yemen since 2015 could lead to a major leakage, causing a major environmental and economic catastrophe. Last year, Houthi authorities granted approval for a United Nations plan to visit and assess the floating storage and offloading unit (FSO) Safer, an oil tanker moored off Yemen's coast with 1.1 million barrels of crude oil. However, on Tuesday, UN spokesman Stephane Dujarric told reporters that the Houthis have not responded to the multiple requests for a letter with the security assurances that the UN needs to facilitate the leasing of "technically equipped service vessels" that are required for the mission. Dujarric explained that without the letter, the cost of the mission would increase by "hundreds of thousands of dollars." "We are also very concerned by indications that the Houthi de-facto authorities are considering a review of their formal approval of the mission to deploy," he said, adding that "Houthi officials have advised the UN to pause certain preparations pending the outcome of such process, which would create further delays to the mission." The vessel's examination team and equipment were scheduled to arrive at the tanker by late January, early February, however last Wednesday Dujarric said the team's arrival date had been delayed until early March. Safer is moored off the Ras Issa oil terminal, 60 kilometres north of the port of Hudaydah; an area held by the Houthis. According to UN officials, water has entered the Safer's engine room at least twice since 2015. Last Thursday, the Yemeni government accused the Houthis of backing away from their commitment regarding the oil tanker. In July Mohammed Ali Al-Houthi, a senior member of the Supreme Political Council said a maintenance team had been dispatched to the tanker but the Saudi-led coalition had blocked access to the necessary equipment.