The sinking of a bulk carrier carrying thousands of tonnes of fertiliser off Yemen after a Huthi missile attack poses "no immediate danger", an expert said Friday.

The Belize-flagged, Lebanese-operated Rubymar sank in the Red Sea on March 2 with 21,000 metric tonnes of ammonium phosphate sulfate fertiliser on board, according to US Central Command.

It had been taking in water since Yemen's Huthi rebels hit it in a missile strike on February 18, damaging its hull and causing an oil slick from leaking fuel.

Concern has grown over the Rubymar now posing a new set of environmental threats under water.

But Christophe Logette, the director of the France-based accidental water pollution management centre Cedre, said there was no cause for instant alarm.

"At this stage there is no immediate danger," said Logette, who is part of a team mandated by the United Nations to assess the effects of the sinking.

"The ship is on the seabed, the hull is in relatively good shape," he said.

The Rubymar went underwater south of the Hanish Islands, a Yemeni archipelago in the southern Red Sea.

The main concern is the fate of the thousands of tonnes of fertiliser, Logette said.

But so far they are "in their storage compartment and there is no trace at the moment of this product being released into the sea".

There had been "no leak" from the holds containing around 200 tonnes of propulsion fuel and 80 tonnes of diesel either, Logette added.

The fear, he explained, is that if any of the fertiliser were to seep out, it would dump a huge amount of nitrate into the water, causing massive algae blooms "which would choke marine life".

But "water has probably filtered into the hull and the cargo. The fertiliser will be wet and so will dissolve very slowly in very low concentrations, with a restricted effect on the marine environment."

The Iran-backed Huthis have repeatedly carried out attacks on shipping since November, saying they are acting in solidarity with Palestinians during the latest war in Gaza.

Palestinian militant group Hamas carried out an unprecedented attack on Israel on October 7, resulting in about 1,160 deaths, most of them civilians, according to Israeli official figures.

Militants took about 250 hostages, some of whom were released during a week-long truce in November. Israel believes 99 hostages remain alive in Gaza and that 31 have died.

Israel has responded with a relentless offensive that the health ministry in Hamas-run Gaza says has killed at least 30,878 people, mostly women and children.