Yemen's Houthi rebels said Wednesday they are "ready" to allow a UN mission to inspect a long-abandoned fuel tanker which threatens to cause a massive oil spill, denying UN allegations of new delaying tactics.

The 45-year-old fuel vessel, abandoned near the western port of Hodeida since 2015, has 1.1 million barrels of crude on board, and a rupture or explosion would have disastrous environmental and humanitarian consequences.

"There is nothing new, no problems and no delays,” Ahmed Dares, a Houthi official responsible for oil affairs, told AFP.

He said they were "ready for maintenance operations” to commence in March, as previously announced by the United Nations.

UN spokesman Stephane Dujarric said on Tuesday that they had yet to receive documents needed for the mission to inspect and repair the FSO Safer, initially planned for early last year and repeatedly delayed.

"To facilitate the leasing of technically equipped service vessels, which are required for the mission, we have requested the Houthi de facto authorities to provide a letter with security assurances,” he said in a statement.

"We regret that, to date, we have not received a response to our multiple requests for this letter, the lack of which would increase the cost of the mission 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.”

Dujarric said that Houthi officials had told the UN to pause certain preparations for the mission, for which the world body had allocated $3.35 million, while they consider a review.

"In light of these challenges, the timeline of deployment of the mission remains uncertain,” he said.

The Houthis confirmed they had agreed to the mission in November last year. The Iran-backed rebels — who control much of Yemen’s north after a war that erupted in 2014 — had previously denied access to the vessel.

Dujarric has also said the UN was reviewing the situation concerning the US designation of the Houthis as a terrorist organisation "to ensure that non-staff contractors are not inadvertently exposed to legal risks by participating in the mission”.

New US President Joe Biden’s administration has suspended the designation until February 26.

The UN has said an oil spill from the FSO Safer could be "catastrophic”, destroying Red Sea ecosystems, shutting down the fishing industry and closing Yemen’s lifeline Hodeida Port for six months.

Independent studies show it could affect countries as far away as Djibouti and Eritrea as well as Saudi Arabia, and expose more than 8.4 million Yemenis — many already vulnerable due to famine after years of war — to heightened pollution.

Apart from corrosion to the ageing vessel, essential work to curb explosive gases in its storage tanks has been neglected