UN humanitarian chief Jan Egeland has accused Sudan of fuelling the worsening conflict in the Darfur region.
The conflict has killed or displaced hundreds of thousands of people
He said the number of people in "desperate need" of aid in Darfur had risen to 4 million, compared to 1 million two years ago.
Mr Egeland said Sudan's government was obstructing international aid efforts and "arming to the teeth" Arab militias accused of attacks on Darfur villagers.
Only a "change in will" in Sudan and abroad could improve matters, he said.
Mr Egeland made his comments in a report to the UN Security Council following his fourth visit to Darfur in his current capacity.
He is expected to step down next month.
The crisis in Darfur was "closer to the abyss than I have witnessed since my first visit in 2004", Mr Egeland said.
"I return with a plea from beleaguered Darfurians for immediate action to finally stop the atrocities against them," he said.
'Wall of obstacles'
Mr Egeland said the number of refugees in urgent need of humanitarian aid had increased "in a climate of massive re-armament".
An international relief operation could only reach 3 million people of the 4 million who now needed aid, he said.
"The Arab militias are being armed to the teeth by the government," he said, while rebels fighting them are "getting arms across the border".
He said the problem had been worsened by a "wall of administrative obstacles that the government has slowly but surely rebuilt both in Khartoum and in Darfur".
More than 200,000 people have died in three years of conflict in Darfur and millions have been displaced.
The refugees are mostly black Africans whose villages have been attacked by militias made up of nomadic Arab herdsmen.
Khartoum has been accused of backing the militias to put down an uprising by Darfur's rebel groups in 2003.
Sudan denies the accusation.
The 7,000 African Union peacekeepers in Darfur are set to be reinforced but Sudan refuses to let the UN take control of the peace mission.