The United States is prepared to move to a "Plan B" for dealing with Sudan if no agreement is reached by Jan. 1 on sending U.N. peacekeepers to Darfur and other steps to end the suffering there, presidential envoy Andrew Natsios says.

"We need to put a time limit on where this is going," Natsios said Monday, declining to describe what consequences Sudan would face if the deadline is not met.

"Making threats is not a wise thing to do," he said.

Natsios told a news conference the deadline is not arbitrary because the mandate for the 7,000-member African Union peacekeeping troops in Darfur expires on that date. He also noted that U.N. Secretary-General Kofi Annan, who has made peace in Darfur a top priority, is stepping down on Jan. 1.

Official optimism about a final settlement has risen somewhat since the Sudanese government joined with the United Nations and the African Union in a framework agreement that included some concessions by the Sudanese, including support for U.N. assistance proposals.

But Sudan has yet to agree to the proposed deployment of a "hybrid" force of 20,000 United Nations and AU peacekeepers and police officers.

Undiscouraged, Natsios said he knows how the Sudanese operate after 17 years of dealing with them. "You frequently will take two steps forward and one step back," he said. The framework agreement was reached last Thursday at a meeting in Ethiopia.

"Our goal here is to get the Sudanese government to negotiate an agreement that they will then carry out with the United Nations that will result in a force, a hybrid force, going to Darfur," Natsios said.

The one issue that is not subject to negotiation is perceived Sudanese government participation in atrocities in Darfur, Natsios said.

"Human rights abuses are not negotiable," he said. "There is no compromise on that."

Susan Rice, a top Africa aide in the Clinton administration, has assailed the U.N.-AU plan as a "colossal sellout."

"We have a fig leaf here that won't solve the problem," Rice said last week. She added that it was unseemly for the international community to be "negotiating with the perpetrators of genocide."

Sudanese refusal to accept a "robust" international force should be met with a U.S.- and European-led bombing campaign against Sudanese airfields and other targets, Rice said.

Natsios said he was especially pleased by the "very helpful" role played by Chinese Ambassador Wang Guangya in Ethiopia. Since the United Nations first became involved in the Darfur crisis in 2004, China has been widely seen as an advocate of the Sudanese position on Darfur, based on commercial ties.

Natsios, who attended the meeting in Ethiopia, also said Arab League delegates and Egyptian Foreign minister Abul Geit made positive contributions.

A former chief of the U.S. foreign aid program, Natsios has remained relatively silent about his Darfur duties since his appointment by President Bush in September.

On Monday, however, he was very much in the spotlight, appearing at a two-hour think tank forum in the morning, meeting with reporters in the afternoon and presiding at the official opening of an exhibit at the U.S. Holocaust Memorial Museum in the evening.

The museum planned to project wall-sized images of what it described as the "escalating genocide" in Darfur, where more than 400,000 people have died and more than 2 million have been uprooted from their homes in sectarian violence since 2003.