N'DJAMENA, CHAD — Chadian rebels renewed their assault on the capital of this oil-rich central African country Monday, and tens of thousands of people fled as gunfire crackled and artillery shells exploded across the city.

The third day of fighting in N'Djamena threatened to further destabilize an already violent swath of Africa that is home to hundreds of thousands of refugees and borders Sudan's war-ravaged Darfur region.

The United States has abandoned its embassy in Chad, evacuating all but four diplomats who are now stationed at the N'Djamena airport amid heavy fighting between government forces and rebels in the capital.

The downtown embassy, which was hit by indirect fire during weekend clashes, is now vacant and unprotected and the State Department on Monday warned the rebels not to enter the compound, which remains sovereign U.S. territory.

Hours after the rebels went back on the attack following an overnight retreat, the U.N. Security Council authorized France and other nations to help Chad's government.

France has 1,800 soldiers backed by fighter jets based in its former colony, but French officials said there were no immediate plans to take on the insurgents. Referring to the U.N. authorization, Foreign Minister Bernard Kouchner said, "I hope that we won't have to use it."

There were fears of a wider regional conflict. Chadian officials have repeatedly accused Sudan's government of supporting the rebels, and one senior general threatened to attack Sudan in retaliation. Sudan's leadership denied involvement.

The fighting in N'Djamena was believed to have taken a heavy toll. Bodies lay on the streets and the hulks of burned out tanks and other vehicles stood abandoned.

Some 1,000-1,500 insurgents equipped with pickup trucks mounted with machine guns arrived on the city's outskirts Friday after a three-day push across the desert from Chad's eastern border with Sudan. They entered the city early Saturday, reportedly trapping President Idriss Deby in his palace.

Government soldiers launched a fierce counterattack Sunday, strafing rebel positions with helicopter gunships and bombarding them with tank cannons.

By early Monday, the rebels had pulled back, but they insisted it was a tactical withdrawal to give civilians a chance to get away — and the battle resumed in the afternoon.

"Fighting and shelling has started again in N'Djamena," said Helene Caux of the U.N. refugee agency.

The death toll was not known, but military and humanitarian officials estimated casualties were numerous.

"The fighting was heavy, the weapons used were heavy," said a French military spokesman, Capt. Christophe Prazuck. "Probably many people were injured or killed."

Isabelle Defourny, head of Chad operations for the French organization Medecins Sans Frontieres, or Doctors Without Borders, said hundreds of people had been reported wounded, most believed to be civilians hit by stray bullets.

She said fighting made it difficult to reach the wounded, but the group's doctors had treated about 70 injured people since Saturday.

Her organization said tens of thousands of people were fleeing N'Djamena, crossing the Chari River into Cameroon. French soldiers used armored vehicles to evacuate French nationals and other foreigners over the weekend.

The State Department said Monday that a little under 100 of the roughly 500 American citizens who had registered with the U.S. Embassy in Chad had left. It also said the embassy staff had been evacuated from Chad, leaving only four U.S. diplomats at the N'Djamena airport.

The capital was chaotic over the weekend, said Christophe Droeven, head of Catholic Relief Services operations who was evacuated Sunday.

"This is a real battle," he said by telephone from Paris. "We saw burnt tanks, burned out cars, people stealing vehicles, some people looting houses, people running with TVs on their heads."

The worst fighting was on Saturday when "for between six and seven hours they were shooting around us with everything — tanks, choppers, heavy machine guns," Droeven said.

The violence endangers a $300 million global aid operation supporting millions of people in Chad. The U.N.'s World Food Program said Monday that the fighting could disrupt delivery of food to 420,000 Darfur refugees and Chadians displaced by violence.

At the United Nations, the Security Council strongly condemned the rebel attack and called "upon member states to provide support" to Chad's government.

There was no immediate reaction from Paris. But before the council acted, French President Nicolas Sarkozy said France and the European Union could send troops with U.N. approval.

"We must avoid a conflict in Chad, by supporting the legitimate government," Sarkozy told reporters during a visit to Romania. "In no region should weapons be a way to come to power."

Deby himself rose to power just as the rebels are trying to, at the head of an insurgent force that captured N'Djamena in 1990. He has won two elections since then, neither of which was considered free or fair, but has enjoyed strong French support.

The rebels are a coalition of three groups whose leaders include Mahamat Nouri, a former Chadian diplomat who defected 16 months ago, and Timan Erdimi, a nephew of Deby who was his chief of staff. They accuse Deby of corruption and embezzling millions in oil revenue.

The rebels also say Deby has not given enough help to ethnic African rebels who have been fighting Sudan's Arab-dominated government for five years. Some of the Darfur insurgents are from Deby's own tribe, the Zaghawa, who are found in both Chad and Sudan.

An aide to Sarkozy said Sunday that Sudan wanted to crush Deby's regime to derail the imminent deployment of a European Union peacekeeping force that is to operate along the volatile border with Darfur.

State Department spokesman Sean McCormack said that the suspected Sudanese support for the rebels was "very worrying" and that Washington had told Sudan's government to end any backing.

The EU force, which has put its deployment on hold until the situation in Chad becomes clearer, is planned to be based in the area around the eastern town of Adre. Rebels said Sunday they had seized the town, but Chad's government said it repulsed the attack.

Adre, near the Darfur border, is a humanitarian hub surrounded by camps with about 420,000 refugees from Darfur as well as Chadians displaced by violence.

The European Union's foreign policy chief, Javier Solana, said Monday that the fighting in Chad would not stop the EU from deploying 3,700 peacekeepers to protect refugees who have fled violence in Darfur.

"Our wish is to maintain the operation," Solana said in Brussels, Belgium. "We took a decision to launch an operation in Chad because we know very well that Chad is very important for the stability of the region."