"The situation on the ground has, in recent weeks and months, deteriorated rather than improved," AU-UN force chief Jean-Marie Guehenno told a conference of the Brookings Institution, a Washington-based independent research and policy institute.
He said the situation in Darfur was "unacceptable (and) ... cannot be allowed to continue as it is."
"No time can be lost," he stressed, adding that the "intensification of the political process needs to happen quickly."
"We need to understand we are on a very tight timeline. Decisions have to be made," Andrew Natsios, special US envoy for Darfur told the conference.
Natsios noted that in January there would be a new UN secretary general and a new US Congress, while the mandate of the African Union mission (AMIS) in Darfur would come to a close.
After long negotiations in Addis Ababa, UN Secretary General Kofi Annan announced last week that Sudan had in principle agreed to a joint AU-UN peacekeeping force in Darfur.
However, on Saturday the Sudanese government rejected the joint peacekeeping operation, favouring instead an extension for the current African Union monitors.
Guehenno called on Khartoum and all other parties involved in Darfur to accept the fact that an effective force was needed for the region.
"There will a need for some non-African troops from traditional peacekeeping countries like the countries of South Asia, which have always made a great contribution to peacekeeping," Guehenno added.
Natsios said the United States supported Annan's efforts and had "no hidden agenda" in Darfur.
The war in Darfur erupted in February 2003 when rebels from minority tribes took up arms to demand an equal share of national resources, prompting a heavy-handed crackdown from government forces and a proxy militia called the Janjaweed.
According to the UN, some 200,000 people have died and 2.5 million more have been displaced in three years of fighting.

Copyright AFP (Agence France-Presse), 2006