Houthis rebels are still blocking access to a deteriorating floating production storage and offloading vessel off Yemen, amid renewed warnings the region faces a growing risk from a rupture or explosion on board.
The United Nations Security Council already believes damage to the idle 407,000-dwt Safer (built 1976) means the FPSO cannot be repaired.
But the organisation wants to get on board for an assessment and then a temporary fix, before deciding the next move.
The council has called on the rebel militia to allow UN experts on board the unit, which is 6 km (3.7 miles) from the port of Ras Isa and carrying 1.2m barrels of crude.
Environmentalists have described the FPSO as a "floating bomb".
Inger Andersen, executive director of the UN Environment Programme, told the council an explosion on the Safer would affect millions of people and a spill would take decades to clean up, al Jazeera reported.
"Economic impacts, social and health impacts would also be dire," she said.
"It is estimated that up to 670,000 people’s livelihoods could be impacted by a spill, and resulting in the damage to fisheries, to marine resources, coastal industries as well as to the economy and food imports."
The Houthis had signalled last July that it would allow technicians on board.
Council members stressed "the need to urgently resolve outstanding issues and called on the Houthis to facilitate unconditional and safe access for UN experts to conduct a comprehensive and impartial assessment and initial repair mission, without further delay".
The Houthis responded in a statement that talks are deadlocked. It blamed the UN.
The group claimed the UN has ruled out most of the agreed maintenance due to lack of funding.
UN spokesman Stephane Dujarric said after the statement was made that the UN had carried out "very intensive discussions" with the Houthis over the past 10 days.
"We're not there yet, which is very unfortunate," he added.
Dujarric claimed the Houthis wants the UN to carry out repairs immediately, which he said cannot be done.
An initial assessment has to be carried out first, he added.
Last year an "oil spot" was seen near the FPSO and an oil leak was reported in December 2019.
The ship has been stranded for six years as both the Houthis and Yemen's Saudi Arabian-backed government-in-exile claim ownership of the $80m cargo.
The UN has warned a spill from the tanker could dump four times more oil than the 1989 Exxon Valdez disaster off Alaska.