"The situation in Darfur is at a crossroads," Ban said in his latest monthly report on implementing a July Security Council resolution authorizing the UN-African Union force.

Ban warned that preparations for the joint UNAMID peacekeeping force to take over from a hard-pressed AU force at the end of this year were being hampered by poor security and because Sudan has yet to approve the composition of the force.

Sudanese President Omar Hassan al-Bashir, who has been accused by the United States and other Western countries of dragging his feet on the issue of peacekeepers, said he was ready for the force to be deployed, but funding was an obstacle.

A UN official said the world body had mechanisms in place to spend money before a budget is officially approved and it was already doing so to support the AU force.  

The main shortfall in contributions to the force is helicopters. No country has offered the 24 transport and attack helicopters needed to help the planned force of up to 26,000 operate in an area the size of France.

Save Darfur, a group of 180 international activist groups, said on Wednesday it had sent letters to the governments of 17 countries urging them to offer helicopters.

The list includes South Africa, as well as Canada, Poland, France, Russia, Egypt, Britain, Turkey, India and Ukraine.