The Sudanese authorities must honour an agreement with the United Nations to end restrictions on humanitarian operations in the western Darfur region, a top United States official has said.

"When it comes to humanitarian access, the government of Sudan’s record is not encouraging," US Deputy Secretary of State John Negroponte said on Monday.

"The denial of visas, the harassment of aid workers and other measures, have created the impression that the government of Sudan is engaged in a deliberate campaign of intimidation."

Negroponte, who visited Darfur on Saturday and "witnessed immense devastation there", said restrictions on humanitarian workers had paralysed aid operations in the war-ravaged region.

He told reporters in the Sudanese capital of Khartoum that the agreement between the government of Sudan and the UN would be "watched closely" to ensure that it is properly implemented.

The agreement, signed last month, aims to cut bureaucratic procedures for aid workers by granting visas and travel permits within 48 hours.

Negroponte’s visit also focused on demands by the international community for Khartoum to allow a sizable UN force to bolster the struggling African Union (AU) mission currently on the ground.

"We must move quickly to a larger, hybrid United Nations-African Union peacekeeping force with a single unified chain of command that conforms to United Nations standards and practices,” said Negroponte.

The Sudanese authorities agreed in November, in principal, to a three-phase UN support package that would culminate in the deployment of about 20,000 peacekeepers. However, to date, Khartoum has failed to fomalise its agreement and has compared UN entry to colonial occupation, demanding that the UN only provide technical and logistical support to the cash-strapped AU.

"There have been disappointments in the past, where agreements have been made, but then not necessarily carried out," Negroponte said. "What I would stress at this particular point is that it is actions that are required and that words are not sufficient."

On Sunday, an unknown gunman shot and killed an AU officer in the north Darfur capital of El Fasher, bringing to seven the number of AU soldiers killed in April.

The Darfur conflict started in 2003 when rebels fighting what they claimed was marginalisation of the region by Khartoum, launched an insurgency. The Sudanese government responded by arming militias to fight the rebels.

Human rights activists, however, say the Arab militias known as the Janjawid, instead turned their guns on civilians, leaving hundreds of thousands dead and several million uprooted from their homes.

On Monday, Oxfam's international director, Penny Lawrence, described the situation in Darfur as the world's greatest humanitarian crisis.

"This is the greatest concentration of human suffering in the world," Lawrence said. "The international community has allowed the conflict to spread, blighting the lives of some four million people and forcing many to the very brink of survival … This is an outrage that affronts the world's moral values."