David Shearer says South Sudan 'dangerously desperate'

Former Labour leader David Shearer says the current situation in South Sudan is worse than anything he's seen in his 26 years delivering aid.

He left politics in New Zealand to head the United Nations mission to South Sudan where more than a 100,000 people are in imminent danger of starving to death - and many millions more are caught up in the crisis.

Civil war and drought are the deadly combination gripping South Sudan and Mr Shearer says this humanitarian crisis is his hardest challenge yet.

"The conditions here are the most difficult I've ever come across, in terms of logistics and just being able to negotiate access," Mr Shearer says.

"And right now one third of the country - 11.5 million people - are separated from their homes, that makes it a desperate situation."

Mr Shearer's job as head of the UN mission is to get aid to those who need it most, like the residents of Maar, north of the capital Juba, who haven't seen food supplies for six months.

Here they're facing not just famine, but a cholera outbreak too, as many flee the war.

"We were sleeping and then the war came to us," witness Ujiwami Beil Bol told CBS. "I saw the soldiers shoot my children and burn my house to the ground."

Heavily pregnant, she walked for 24 days with her two surviving children to find safety, giving birth to a healthy baby along the way.

Mr Shearer says delivering aid in South Sudan is also incredibly complex and slow.

"To get to the most northern area that we need to get to from Juba is a 1000km trip, which if you were going down a motorway in any western place would take you about 10 hours. Here it takes two weeks," he says.

Along the way there are almost a hundred checkpoints - a multitude of government, rebel and ethnic forces all stalling progress, all demanding payment.

"As the UN we don't pay to get through check points, if we did we'd have 1000 checkpoints, so we have to negotiate our way through it and it's a minimum of two weeks, sometimes it can be three or four weeks," Mr Shearer says.

Air dropping supplies avoids the road blocks and gets staple supplies like beans to a desperate community.

But even after the aid drop, Ujiwami Beil Bol struggles.

"I don't know anyone here. I just sit under a tree and hope people will help me," she says.

But there are so many mouths to feed that she misses out.