The head of the United Nations children’s agency, Unicef, says Sudan is one of the worst places in the world for children.

Catherine Russell says it now has the largest displacement of children anywhere, with millions facing malnutrition and most not in school

She is travelling to the country torn apart by more than a year of brutal civil war as warnings of famine grow louder.

The pillars of Sudan's food economy have collapsed, and both warring parties - the Sudanese army and a paramilitary group called the Rapid Support Forces (RSF) - are restricting the delivery of desperately needed aid.

Children were at the sharp end of this hunger crisis, Ms Russell told the BBC while en route in Nairobi: nine million don’t get enough to eat regularly, and nearly four million face acute malnutrition.

“We’re long past time where we need to act, we need to act now or it’s just going to get worse,” she said.

“You can always eventually make progress on something, so nothing is completely impossible. But for individual babies, for children, who are starving now, who are hungry, who are now severely malnourished, it will be too late for them.”

Earlier this month, the BBC spoke to a food security expert who said that by September, 70% of Sudan's population will be extremely hungry.

"That could lead to two-and-a-half million deaths, or more" Timmo Gaasbeek said. "It could be as many as four million. There is just not enough food."

The Sudanese Armed Forces and RSF, have divided the country roughly in two, and plunged it into a humanitarian catastrophe.

The army has restricted access for food deliveries across conflict lines into areas controlled by the RSF.

RSF fighters are accused of widely plundering resources and have been besieging a city of nearly two million people, El Fasher, for more than a month.

Ms Russell said she couldn’t speak to whether they were using hunger as a weapon of war. But she said the crisis was “100% man-made.”

“The challenge for us is not that we don't have the food, it’s that we can't get it to the people who need it. And that is really a crisis.”

Ms Russell said Sudan had the highest number of displaced children in the world – five million – and nearly all of its children were out of school, in danger of becoming a lost generation that could contribute to future instability.

“It's hard to re-teach them, because that's a lot of lost learning. But it's also hard, in many cases, to get them back into the classroom," she said.

"So in that sense, they can become lost… And if you lose that, what do we think the future is going to be like? It's going to be unstable.”

She will add her voice to the chorus of demands for an end to the fighting. But, a recent UN appeal for calm in El Fasher was ignored, and US efforts to restart peace talks have so far failed.

“There’s limited bandwidth” she said when asked about the lack of persistent international engagement to wrest order from the growing chaos in the strategic African country, citing the conflicts in Gaza, Ukraine and Haiti.

“That's really the reason I'm going, to try to draw some attention to it and say, we need to focus on this right now. This is quite dire.

"And if we don't do something, it's hard to imagine how bad it will be.”