"The deployment of UNAMID [United Nations African Union Mission in Darfur] is facing fundamental challenges in the areas of force generation, the final identification of the force composition and bureaucratic impediments," French head of the UN peacekeeping operations Jean-Marie Guehenno said, in a prepared statement at a Security Council debate.
He criticised Sudan for blocking the inclusion in the force of certain non-African units, from countries including Thailand, Nepal and nordic nations.
Its refusal creates "serious uncertainty with regard to the Government's commitment to the deployment of UNAMID," he said.
Since a war over Darfur's resources broke out four years ago between poor black tribes and government-backed Arabs, more than 200,000 people have died from fighting, famine and disease while 2.2 million others have been left homeless, the UN says.
It agreed in July that a joint UN-African Union (AU) peacekeeping mission of 26,000 troops would take over from some 6,000 ill-equipped AU troops operating in an area the size of France since 2004.
Sudan long resisted approving a force, which it finally accepted on condition it be mostly composed of African troops.
But AU and UN officials said this month they were struggling to get the troops needed for their joint force, named UNAMID, which is due to start replacing the AU deployment from the start of 2008.
"Five weeks before the transfer of authority, UNAMID is still short of critical mobility capabilities," such as helicopters, Mr Guehenno said on Tuesday (local time).
The UN's top political envoy for Darfur, Jan Eliasson, also painted a dark picture of the prospects for peace between the government and rebels.
"There is continued reason to be concerned by the present security and humanitarian situation, which clearly affects the political process," Mr Eliasson said.
Peace talks began on October 27 but have since been boycotted by several rebel groups.
"The atmosphere now is less positive than it was last summer," Mr Eliasson added.
"We will only be able to make progress if the parties show seriousness, political will and a focused commitment to peace." © 2007 Australian Broadcasting Corporation