The United Nations humanitarian chief warned on Friday that the fight against famine is being lost in Yemen, which is already facing the world's worst humanitarian crisis with 75 per cent of its 29 million people in need of assistance.

Mark Lowcock called the situation "bleak" and told the Security Council it "has deteriorated in an alarming way in recent weeks".

"We may now be approaching a tipping point, beyond which it will be impossible to prevent massive loss of life as a result of widespread famine across the country," he said.

"We are already seeing pockets of famine-like conditions, including cases where people are eating leaves."

Lowcock said two recent developments threaten to overwhelm the aid operation — a "dramatic economic collapse" that has reduced the value of Yemen's currency by some 30pc, and intensified fighting around the Red Sea port of Hodeida, which is key to deliveries of food, medicine and other vital supplies.

The conflict in Yemen, the Arab world's poorest country, began with the 2014 takeover of the capital, Sanaa, by Houthi rebels, which toppled the internationally recognised government.

A Saudi-led coalition allied with the government has been fighting the Houthis since 2015.

Civilians have borne the brunt of the conflict, which has killed over 10,000 people and sparked a cholera epidemic.

Lowcock said that during the first six months of this year, the UN and humanitarian groups provided assistance to more than 8m of the most vulnerable Yemenis who don't know where their next meal will come from.

That is a dramatic expansion from 2017, when food was reaching 3m people a month.

Because almost all Yemenis rely on imported food, Lowcock said, the currency depreciation "translates directly into a sharp increase in the price of food for some 10m Yemenis" who aren't getting enough food but aren't part of the aid operation.

There has also been an "unprecedented increases" in the price of fuel, he said.

In addition, Lowcock said, intensified fighting in recent weeks around Hodeida is "choking the lifeline" for getting aid to those in need.

Fighting in recent days has cut the main road from Hodeida to Sanaa, which is the principal conduit for both commercial importers and aid groups for moving supplies, he said.

Other routes are heavily damaged, he said.

Lowcock said armed groups have occupied humanitarian facilities, and attacks have resulted in dozens of deaths and serious damage to public health and water facilities as well as other aid infrastructure.

"We estimate that an additional 3.5m people may soon be added to the 8m already severely food insecure," he said.

Lowcock said the UN continues to push to scale up its aid operation, but "humanitarian organisations simply cannot look after the needs of all 29m Yemenis. That is untenable".

He urged the Security Council to press for immediate measures to stabilise Yemen's economy and support the exchange rate, including ensuring liquidity for the central bank and implementing longstanding commitments to pay salaries of teachers, doctors, health workers and other public service employees throughout the country.

Lowcock also urged that ports be kept open, warning that "the lifeline through which the aid operation runs now hangs by a thread".

He called for protection of civilians, schools, hospitals and other civilian infrastructure, the establishment of "an air bridge" for medical evacuations and serious negotiations on "a positive path towards peace".