Iran and the United States should "immediately" allow fertiliser shipments through the Strait of Hormuz to ease the crisis in some of the most vulnerable countries, a senior UN official said Saturday.

Jorge Moreira da Silva, head of the UN Office for Project Services, told the COP31 climate talks in the Turkish resort city of Antalya that the lack of fertilisers could push millions into poverty.

While Iran and the United States are still trying to negotiate an end to the conflict, Moreira da Silva said he had the same message for both countries.

"They should continue their negotiations about the comprehensive deal - but that deal should not avoid an immediate, exceptional, limited, and time-bound solution for the fertilisers," he said.

"Basically what we need is five vessels a day with fertilisers," he added.

Sudan's finance minister recently told him that the price of food there had doubled in six months, he added. In Nepal, they had told him that farmers could no longer afford to buy fertilisers.

"So it's very important that we are aware that countries such as Mozambique, Kenya, Sudan, Somalia, Nepal, Bangladesh, Sri Lanka are facing very significant impact."

The effects of this shortage could be catastrophic, he warned.

"If we continue missing the planting seasons in different parts of the world we may have 45 million people facing severe and acute food insecurity," he said.

"We may have 30 million people being forced into poverty."