By Paul Reynolds
World affairs correspondent, BBC News website
The UN fears aid distribution could become impossible
The expulsion by the Sudanese government of the UN envoy Jan Pronk is another set-back to hopes of persuading the Sudanese to accept UN peacekeepers in Darfur.
A few weeks ago, western governments began an effort to put pressure on Sudan to accept Security Council resolution 1706, passed on 31 August. This called for a force of 20,000 soldiers and police. In theory, the resolution has to be accepted by Sudan. In practice it has not been.
China was asked to help, given that China has extensive economic interests in Sudan, especially oil development.
At the time the British Minister for Africa Lord Triesman declared: "At the end of this period, in which florid rhetoric is being used, Sudan's position will be impossible to sustain."
However Sudan's position has so far been sustained.
Among the florid rhetoric used was a comment by Sudanese President Omar Bashir about international groups that raise the Darfur issue. He said: "Those who made the publicity, who mobilised the people, invariably are Jewish organisations."
What has happened since the western campaign began is that the mandate of the weak force of 7,000 African Union troops has been extended until the end of the year.
Jendayi Frazer visited President Bashir on behalf of President Bush
This is a stop-gap measure. The UN plan is to build on that force by adding in soldiers from other countries to make it more effective.
President Bush said on 2 October, when he appointed Andrew Natsios as his special envoy to Sudan: "In my view, the United Nations should not wait any longer to approve a blue helmeted force, a UN force of peacekeepers to protect the innocent people."
Sudan has rejected a UN force on the grounds that it would be like an invasion. It also warned that Darfur could become a new battlefield for jihadists, as Osama bin Laden had stated that any UN intervention should be resisted.
Lord Triesman accepted that even if Sudan held to its view and did not accept the UN force, it was "not realistic to invade Sudan".
In the meantime, fighting continues despite a peace agreement reached in Nigeria in May. Only one of three rebel groups in Darfur accepted the agreement and the Sudanese government said it would send in forces to combat them.
Mr Pronk upset the Sudanese government on his personal website by saying recently that the armed forces had suffered defeats in Darfur. He also angered the government by meeting members of a group associated with the National Redemption Front which has rejected the peace agreement.
Throughout this crisis, Sudan has managed to stave off UN intervention and it is not yet clear whether the UN force will ever get in.
The United States insists that because Security Council resolution 1706 is written under the enforcement chapter of the UN, the force could go in regardless of what the government of Sudan wants, but obviously that is not the preferred option.
The US Assistant Secretary for African Affairs, Jendayi Frazer, went to see President Bashir to deliver a message from President Bush, urging Sudan to allow the UN force in.
In a briefing on her return to Washington on 31 August, she said the resolution was "the key step to ultimately ending the crisis in Darfur, and the United States continues to support strengthening the African Union force in Darfur and having those troops become the core of a UN mission in Darfur".
Since the start of the crisis, Sudan has managed to avoid having an effective peacekeeping force in place - and it is still playing that diplomatic game.
Darfur has found itself a crisis that neither the UN nor the relatively new African Union can solve. The UN has lacked the will to intervene and the African Union has lacked the means.
Despite the statement from the then US Secretary of State Colin Powell in September 2004 that genocide had taken place in Darfur, a UN commission that reported in January 2005 concluded that while there had been crimes against humanity and war crimes, the government of Sudan "had not pursued a policy of genocide". A finding of genocide would have forced the UN to intervene more strongly.
Yet the scale of suffering is huge. Tens of thousands, perhaps 200,000, are reckoned to have died and the UN commission said 1.65m people had been forced from their homes, with another 200,000 as refugees in Chad.