The U.N. Undersecretary for Humanitarian Affairs has warned that the massive aid operation sustaining Sudan's war-torn Darfur region is on the verge of collapse if insecurity in the region continues. Noel King has this report from Khartoum.

Top U.N. envoy John Holmes on Monday warned that humanitarian aid to Darfur has been endangered by continued insecurity and called for an end to restrictions placed on aid workers by the Sudanese government.

Holmes spoke to reporters in the Sudanese capital Khartoum, following a two-day visit to Darfur.

"If we do not nurture the partnerships between members of the humanitarian community and the government; and if the problems get worse this humanitarian effort could begin to unravel with possibly catastrophic consequences for the population," he said.

Holmes faced restrictions during his visit.

The U.N. humanitarian chief was barred by Sudanese security at a military checkpoint from visiting the Kassab IDP camp in northern Darfur.

Darfur refugees, sitting under a make-shift shelter in this derelict section of Es Sallam camp, say they have been waiting nine months to be relocated to decent shelters, 25 Mar 2007

The governor of north Darfur state later apologized to Holmes.

A series of violent attacks on aid agency compounds late last year by unknown armed assailants caused a massive withdrawal of humanitarian workers across Darfur.

Some agencies have not yet returned to the region.

Sudan has also been accused of making humanitarian staff undergo lengthy procedures in order to obtain visas and permits to work in the region.

Sudan is charged with arming the janjaweed militias to crush a rebellion by African farmers that began four years ago.

Experts estimate 200,000 people have died and more than 2.5 million others have been displaced.

Holmes' visit to Sudan was the first leg of a three-nation tour that will also include trips to Chad and the Central African Republic.

Observers have warned that violence in Darfur is spilling over into those neighboring countries.