The United States has warned Sudan that any support it may be giving to rebels in Chad is "unacceptable."
Speaking to reporters Monday, U.S. State Department spokesman Sean McCormack said he cannot offer a "definite, public conclusion" as to whether Sudan is arming or aiding the rebels.
But the spokesman added that he cannot wave off that possibility.
He also said the United States has made it very clear to the Sudanese government that such behavior is unacceptable.
Chad's President Idriss Deby has accused Sudan of supporting the rebels who oppose him and attacked the capital, N'Djamena last week. Sudan denies the allegations.
Meanwhile, the United Nations refugee agency says it has no immediate plans to evacuate more staff from Chad.
A spokeswoman said Tuesday, that the agency recently flew some 35 staffers to Cameroon but wants to strike a balance between protecting staff and continuing essential services.
The agency runs camps for some 200,000 Sudanese refugees in eastern Chad who fled Sudan's Darfur conflict.
Last week, President Deby threatened to expel the refugees. But the U.N. High Commissioner for Refugees says he has received assurances from Mr. Deby that the refugees can stay.