He said that if the predominantly African 26,000-strong hybrid force "is not able to meet that test in a credible manner, then it will be  very bad for our efforts in Darfur".
"The clock is ticking" ... we are missing some critical  capacities," he said, referring to 24 transport or attack helicopters.
Lack of confidence
Diplomats have said several Western countries able to provide the helicopters are reluctant to do so because of a lack of confidence in the the command-and-control structure for the joint force.
Another key unresolved issue, Guehenno said, was Sudan's apparent foot-dragging on allowing the deployment of four key non-African units for the force: a Thai infantry battalion, two Nepalese special forces contingents and a Scandinavian engineering  unit.
"Each of those units has particular importance for the force.  It's essential for those to deploy early ... for force protection,"  the UN official said."
They are ready to deploy in early 2008 and if they don't there is no alternative."
But Guehenno said Khartoum had yet to state clearly whether it will allow those non-African units to deploy.
Security Council briefed
Guehenno spoke to reporters after briefing the 15-member UN Security Council on latest preparations to deploy the joint force known as  Unamid.
A September 29 attack on an AU base in south Darfur left 10 African peacekeepers dead and drew worldwide condemnation.
The force is charged with ending more than four years of  bloodshed in which more than 200,000 people have died from the  combined effects of war, famine and disease while 2.2 million others  have been left homeless.
Recently, regional authorities in Darfur expelled a UN humanitarian official, accusing him of unspecified rule violations.
Wael al-Haj-Ibrahim headed the UN office for the Co-ordination of Humanitarian Affairs (OCHA) in the town of Nyala, which was in charge of aid for up to one million displaced people.