The United Nations has launched a crowdfunding appeal to raise some of the $20m needed to lighter oil from a decaying floating storage and offloading (FSO) vessel off the coast of Yemen.

The social media campaign opened for public donations on Tuesday following the failure to hit the $80m target needed to empty the 407,000-dwt FSO Safer (built 1976) that is at risk of leaking and causing an environmental catastrophe.

The ship has about 1.1m barrels of crude on board – four times the amount of oil spilled by the Exxon Valdez in 1989. It has not been maintained since 2015, when an offensive by Houthi rebels forced the country’s president to flee.

The UN is seeking $144m to remove the oil and a second longer-term target of replacing the rotting Safer with a new vessel. It has raised $60m for the first stage of the operation but needs another $20m.

Officials hope that the crowdfunding campaign can raise a quarter of that sum by the end of the June, shaming countries and the private sector into coming up with the rest.

David Gressly, the UN’s resident coordinator for Yemen, said the project was “very far advanced” on procurement of the salvage operation.

“Frankly speaking, the primary constraint we face is no longer really political, security, procurement or operational. It’s resources,” Gressly said on Monday.

A funding conference in May took the total to $40m while the US and Saudi Arabia – which led the coalition fighting the Houthi rebels in Yemen – each contributed a further $10m.

Mr Gressly said time was running short to raise funds for the first stage of the operation with worsening winds in October and November increasing the chances of the old and decaying vessel breaking up 4.8 nautical miles (8.9 km) off Yemen’s Red Sea coast.

The UN says a leak would cause a major humanitarian and environmental disaster that would hit Yemen hardest but with the potential for oil to drift to neighbouring countries including Djibouti, Eritrea and Saudi Arabia.

“$20m is really not much when you look at the overall cost that his catastrophe would have,” he said. “If indeed there were a spill, the estimates that we’ve received on the cleanup alone would be $20bn.”