United Nations agency chiefs on Friday demanded unimpeded humanitarian access to deliver aid throughout war-torn Sudan, saying time was running out to prevent widespread famine.

In a joint statement, the heads of multiple UN agencies urged all parties in the conflict to immediately stop denying and obstructing humanitarian actions.

"Let us be clear: If we are prevented from providing aid rapidly and at scale, more people will die," the statement from the UN Inter-Agency Standing Committee said.

"Without an immediate and major step change, we will face a nightmare scenario: A famine will take hold in large parts of the country. More people will flee to neighbouring countries in search of sustenance and safety. More children will succumb to disease and malnutrition."

The joint statement was penned by the UN aid chief Martin Griffiths, along with the heads of the UN agencies for food, health, human rights, refugees, migration, agriculture and children, among others.

The committee is the highest-level humanitarian coordination forum in the United Nations system, bringing together the heads of 19 organisations, some of which are from outside the UN.

Fighting in Sudan broke out in April last year between the regular army and the paramilitary Rapid Support Forces. The conflict has killed thousands and sparked a humanitarian disaster.

The statement said 18 million people were acutely hungry, 3.6 million children were acutely malnourished and famine was closing in on millions of people.

Nearly two million people have escaped to neighbouring countries while millions more are displaced within Sudan.

  'Tremendous needs' 

 "Despite the tremendous needs, aid workers continue to face systematic obstructions and deliberate denials of access by parties to the conflict," said the statement from the UN Inter-Agency Standing Committee.

"Aid workers are being killed, injured and harassed, and humanitarian supplies are being looted."

The statement said movement across conflict lines to parts of Khartoum, Darfur, Al-Jazira and Kordofan had been all but cut off since mid-December.

The UN agency chiefs made a series of demands, including an immediate ceasefire, protection for civilians and an end to human rights violations.

"Facilitate unimpeded humanitarian access through all possible crossline and cross-border routes to allow civilians to receive humanitarian aid. Immediately cease all acts denying, obstructing and interfering with, or politicising, humanitarian action," they said.

"Simplify and expedite administrative and bureaucratic procedures related to the delivery of humanitarian aid."

Jens Laerke, spokesman for the UN humanitarian agency OCHA, told a media briefing in Geneva that the conditions for aid delivery were "very, very poor, and it's very, very dangerous".

"We want these generals to find a way to solve their differences not by violence that kills, maims, rapes hundreds of thousands of people," he said.

The UN also said it was concerned by limited donor support, having received only 16 percent of the $2.7 billion it needs for Sudan.