The Sudanese government is "paralysing" the aid operation in its conflict-torn western region of Darfur, the US special envoy to Sudan has said.
Some two million people live in Darfur's camps
Andrew Natsios said there had recently been a big increase in red tape and the harassment of aid workers.
He was speaking after talks with Sudanese President Omar al-Bashir.
The operation is the biggest of its kind in the world, with about two million people in Darfur's camps fully dependent on aid agencies for food.
"The [Sudanese] government has constructed a very onerous set of bureaucratic requirements which are essentially paralysing the relief effort," Mr Natsios said after the talks in Khartoum.
He said aid workers were facing numerous visa and access restrictions from Sudan's government agencies and security forces.
Mr Natsios also said that in the last few months humanitarian workers had been sexually assaulted and beaten by government workers as well as rebel fighters.
"The greatest immediate threat to the people on the ground is the deteriorating humanitarian space in Darfur," he said.
The US envoy also said Khartoum had created unacceptable delays in the transition to a joint United Nations-African Union peacekeeping force in Sudan's western region.
AU peacekeepers have a mandate to remain in Darfur until June 2007.
Khartoum rejects plans for it to hand over to a larger, stronger UN mission, with President Bashir calling it an attempt by the West to colonise Sudan.
On Wednesday, the AU said two of its peacekeepers had been killed and a third seriously hurt in Darfur.
An AU statement said the soldiers were abducted before being shot in the town of Gereida in the western province.
The attack was carried out by a rebel faction who signed last year's peace agreement, the statement said.
The deaths - the latest in a series of incidents this week - bring to 11 the number of AU peacekeepers killed since their mission began in 2004.
Sudan's government and the pro-government Arab militias are accused of war crimes against the region's black African population, although the UN has stopped short of calling it genocide.
But the current force has failed to halt attacks on civilians which has led to some two million people living in camps.