The UN World Food Program says more than 20 million people in Yemen, north Nigeria, Somalia and South Sudan are at risk of dying from starvation within six months.

Yemen is the worst affected country with the UN describing it as being "on the brink of famine". More than 7.3 million people urgently need food assistance and more than 460,000 children are suffering from severe acute malnutrition.

"The pictures are harrowing, the numbers are amazing," Jamie McGoldrick, OCHA (Office for the Coordination of Humanitarian Affairs) for Yemen said.

"I think it's creating one of the world's worst humanitarian disasters. The possibility and threat of famine is looming."

The starvation in Yemen stems from two years of war that has maimed and killed thousands of civilians.

A Saudi led coalition has been bombing the country since March 2015 to try and get rid of Iranian aligned Houthi rebels who overthrew the Government.

The coalition has enforced a blockade on the poverty stricken country, which is 90 per cent dependent on food imports.

There are now indications food security in Yemen is about to get much worse.

"We've been told now by the coalition to redirect the shipments including the humanitarian supplies to Aden in the south," Mr McGoldirck said.

Potentially devastating effects

This move by the Saudi coalition to stop aid coming through Hudaydah Port, Yemen's main port, could have potentially devastating effects.

Mr McGoldrick said approximately 80 per cent of goods for the north of the country currently come through Hudaydah.

The Saudi coalition is trying to redirect aid and humanitarian supplies from the Hudaydah port. ( Reuters: Abduljabbar Zeyad )

Critical food supplies for the millions of Yemenis at risk of starvation will now have to be shipped to the smaller southern port of Aden and trucked up to Sanaa and other northern towns, crossing through areas of fighting and warring party lines.

"What we are talking about now is increasing those by thousands of trucks a month with food only," Mr McGoldrick said.

"Given that the country is 90 per cent dependent on food imports if this is left as it is we are talking about the onset of famine and mass starvation in this country"

Western governments are now on notice. The warnings of mass starvation are loud and clear.

The UN urgently needs more than $5 billion by the end of March to help the 7 million on the brink of famine in Yemen and the nearly 13 million people facing possible in north Nigeria, Somalia and South Sudan.

"We are facing a tragedy. We must avoid it becoming a catastrophe," UN Secretary General Antonio Guterres told reporters at a press conference in New York.

"This is preventable if the international community takes decisive action."