The United States on Monday warned Chadian rebels fighting government troops in the capital not to enter the U.S. embassy compound in N'Djamena that was abandoned over the weekend for security reasons.

"The embassy compound, if it has been entered, should immediately be exited, and certainly they should not attempt to enter the chancery or embassy buildings," said State Department spokesman Sean McCormack. "It is still American territory."

Over the weekend, the embassy ordered nonessential U.S. staff and the families of all U.S. personnel to leave Chad and only four U.S. diplomats, including Ambassador Louis Nigro, are still in the country. They left the embassy compound on Saturday and are now stationed at the international airport outside N'Djamena.

"The security situation was very fluid," McCormack said, explaining Nigro's decision to relocate to the airport, which is guarded by the French military. "The rebels were downtown and every ambassador has to make this call. Job number one is to make sure that our people are safe."

He said the warning not to enter the embassy was being passed to the rebels "through various channels" but acknowledged that there was little guarantee the compound would not be breached.

"If there is a determined force with heavy armor and they want to go there, they are going to be able to go there, regardless of what sort of lightly armed local protection we might have," he told reporters.

The State Department has urged U.S. citizens to leave Chad, and the embassy staff in N'Djamena has assisted numerous Americans in departing but McCormack could not say how many had gone. It was not immediately clear how many U.S. citizens are in the country because many do not register with the embassy, he said.

Without any personnel at the abandoned embassy, the State Department has set up two telephone numbers to report the presence and whereabouts of Americans who are in Chad. Those numbers are (888) 407-4747 from the United States and Canada and (202) 501-4444 from overseas.

However, on Sunday, the department said its ability to help Americans in Chad was "extremely limited" and asked those who wished to be evacuated to "prepare to depart immediately and identify themselves to the French military, who will retrieve American citizens to escort them to the airport."

France, the former colonial power, has a long-standing military presence in Chad and is evacuating hundreds of foreigners from the country.

On Monday, Chadian government forces clashed with rebels for a third day with gunfire and shelling heard throughout the city. Thousands of people were fleeing and casualties were believed to be high.

The rebels arrived on the capital's outskirts Friday after a three-day push across the desert from Chad's eastern border with Sudan. Riding on 250 pickup trucks mounted with machine guns, between 1,000 and 1,500 insurgents entered the city early Saturday, quickly spreading through the streets and reportedly trapping President Idriss Deby in his palace.