Famine could become part of war-torn Yemen’s "reality” in 2021, the UN warned on Tuesday, after a donor conference raising funds to keep millions from starvation fell short by over half.

The United Nations had sought Monday to raise $3.85 billion from more than 100 governments and donors, but only $1.7 billion was offered.

"The prospect of more people suffering a famine has just grown exponentially,” UN Development Programme Chief Achim Steiner told AFP on Tuesday.

Tens of thousands of people have been killed in the six-year-old war, which pits the Iran-backed Houthi rebels against an internationally recognised government supported by a Saudi-led military coalition.

As fierce battles continue amid a rebel push to seize the government stronghold of Marib, Steiner said Yemen was "a step away” from famine.

"The tipping point...can happen just in a matter of weeks... and that is why we are so concerned we may see a major famine really becoming part of Yemen’s reality in the year 2021,” he said.

Steiner said the "immediate implications” of decreasing levels of support will be devastating, with UN and international aid agencies having to slash support at the "worst moment”.

More than two thirds of the 29 million Yemenis are relying on aid — which has already been cut from last year, putting a freeze on major programmes.

"Rations have to be cut, food supplies can no longer be provided, health supplies and basic medicines will essentially not be available,” Steiner said.

"With the pledges made yesterday, there is perhaps a few more months that have been gained, but the prospect of a severe cutback in food supplies is imminent,” he added.

The term famine is used according to a globally agreed scale determined by levels of access to food, acute malnutrition and daily deaths due to hunger.

While gathering the heavy burden of data needed to formally declare a famine is hard to do, nearly 50,000 are already starving to death in famine-like conditions, according to the World Food Programme.

"It is literally a matter of life and death for many of them in terms of access to food and basic supplies,” Steiner said.

The UN describes Yemen as the world’s worst humanitarian crisis.

Steiner estimates development progress in Yemen has been set back by years because of the war.

"There is an entire generation of boys and girls right now who are essentially growing up without the most basic things that we can provide in the 21st century, which is an education,” said Steiner.

"There are structural and chronic issues now playing out that will take years and years to even recover to a level of development where Yemen once was just a decade ago.”

For Tariq Ramadan, a resident of the rebel-held capital of Sanaa, stopping the war is Yemen’s only hope of things improving.

"All Yemeni people wish for the war to end as soon as possible. so that we no longer need any kind of aid,” he told AFP.