Richard Williamson, who asked U.N. Secretary-General Ban Ki-moon in March to ensure that 3,600 more U.N.-African Union troops were deployed in the west Sudanese region by the end of June, said only 600-650 new soldiers had been added this year.

The total number of peacekeepers in violence-torn Darfur currently stands at just over 9,000. It is meant to rise to a ceiling of 26,000 troops and police by next year.

Williamson told reporters he was "disappointed the U.N. has not been able to meet those modest goals ... The progress in the first six months is absolutely unacceptable."

"The people of Darfur deserve better," he said. "The suffering which they continue to face because of the ongoing insecurity is something the Security Council has sought to respond to, and the U.N. has to do a better job of meeting its responsibilities."

Williamson noted that the United States was training and equipping African peacekeepers for Darfur but said their deployment depended on the capacity to absorb them on the ground, for which the U.N. secretariat was responsible.

He conceded that part of the problem lay with obstacles raised by the Sudanese government, the harsh, arid environment of Darfur, an area the size of France, and the long supply line from Port Sudan.

But he also criticized U.N. officials for not adopting a "pragmatic, flexible, problem-solving approach."

"They've tried to use practices and procedures used elsewhere in easier circumstances," he said.

Williamson said, however, he had high hopes that Susana Malcorra, an Argentinian recently appointed to head the peacekeeping division's department of field support, would improve the situation.

He said Malcorra had "performed brilliantly" in a previous job at the Rome-based U.N. World Food Program and was already taking a "hands-on approach" in New York.

Independent experts say at least 200,000 people have died and some 2.5 million have been driven from their homes since the Darfur crisis erupted five years ago in the wake of a rebellion. Sudan says only 10,000 have died.