The United States appealed to Chadian factions Wednesday to engage in political dialogue amid reports that rebels opposed to the government of President Idriss Deby may be advancing on the capital, Ndjamena.

The United States is expressing deep concern about the security situation in Chad, and urging all the parties there to reject violence and engage in dialogue to resolve the country's political standoff.

The comments from the State Department came amid what a spokeswoman here said were credible but unconfirmed reports that armed rebel groups may be within 100 kilometers of N'Djamena and may attempt to advance on the capital.

A rebel movement which includes soldiers formerly loyal to the government of President Deby has been active for several months. There have been conflicting claims by the government and rebels over control of the central Chadian town of Mongo, east of the capital.

Mr. Deby accuses neighboring Sudan of backing the rebels, a charge the Khartoum government denies.

State Department spokeswoman Julie Reside, who said the United States condemns any resort to violence in Chad, called the situation there "fluid" and said U.S. officials are monitoring it closely.

She said the department is advising Americans to defer travel to Chad at this time, and urging those already in the north African country to exercise extreme caution.

The reported advance by the rebels, calling themselves the United Front for Change, comes only a month before a scheduled presidential vote in which Mr. Deby is standing for re-election, and which opposition parties have said they will boycott.

Mr. Deby came to power in 1990 after himself launching a rebellion from bases in Sudan's western Darfur region.

U.S. officials have for some time been expressing concern about the spillover effects of the three-year-old Darfur crisis in Chad, where tens of thousands of Sudanese have sought refuge.

France, which maintains a 1,200 member military garrison in Chad, has said it will send another 150 troops there because of the latest events. French officials say the troops are on standby to help evacuate foreigners if needed.