An insurgency has swept across much of the poverty-stricken but resource-rich former French colony since December 10, posing the biggest threat yet to President Francois Bozize's nearly 10 years in power and threatening a humanitarian crisis.

The government on Thursday urged Western powers France and the United States to help push back the rebels, though Paris said it would not use soldiers to defend Bozize's government and Washington evacuated its embassy.

The Central African Republic is one of a number of nations in the region where U.S. Special Forces are helping local forces try to track down the Lords Resistance Army, a rebel group responsible for killing thousands of civilians across four African nations.

"Our last chance, our only chance, is dialogue with the rebels," bus driver Jerome Lega said as he weaved through traffic in the centre of Bangui.

Scores of wooden boats piled high with baggage and people crossed the Oubangui River toward Democratic Republic of Congo on the other side, while the main road south away from rebel lines was choked with overloaded vehicles.

Those remaining said they were stockpiling food and water and praying international mediation efforts would convince the insurgents not to enter the city shooting.

"We are hoping that Bangui will not be attacked," said Eugenie Bosso, a woman running a market stall.

In a sign of rising tensions, the government announced a ban on motorcycle taxis in Bangui after nightfall, amid suspicions rebels were using them to infiltrate the city unnoticed.

GROUNDWORK FOR PEACE TALKS

Envoys from across central Africa arrived in Bangui on Thursday to lay the groundwork for peace talks with the rebels, and regional foreign ministers were due to meet in Gabon later on Friday to discuss the crisis.

African Union Commissioner for Peace and Security Ramtane Lamamra said regional leaders were seeking to convince the rebels to send a delegation to Gabon's capital Libreville to hash out a peace deal and end the crisis.

"If that is not successful, of course other options will be considered," he told reporters in the Ethiopian capital Addis Ababa, adding central African states could provide additional troops to reinforce CAR's army against the rebels.

A spokesman for the SELEKA rebel coalition - which said it will oust Bozize unless he honours a previous rebel peace agreement that provided payments and jobs to former fighters - said on Thursday it would halt its advance short of Bangui to allow for the mediation efforts.

A military source and an aid worker said the rebels had advanced to within 75 km (45 miles) of Bangui by late Wednesday, and a diplomatic source said they had since moved closer to the capital, effectively surrounding it.

The rebel advance has highlighted the instability of a country that has remained poor since independence from France in 1960 despite rich deposits of uranium, gold and diamonds. Average income is barely $2 a day.

French nuclear energy group Areva (AREVA.PA) mines the Bakouma uranium deposit in the CAR's south - France's biggest commercial interest in its former colony.

The U.N. Security Council on Thursday condemned the rebel advance. Regional and Western powers have been pushing for a negotiated solution.

Neighbouring Chad has sent troops to bolster CAR's weak army though it is unclear whether they would be enough to halt a renewed rebel assault on the capital.

The International Committee of the Red Cross said it was working to provide displaced people with water, sanitary facilities, and other necessities, and called on rebel and government forces to spare civilians. It said it had withdrawn eight staff for security reasons, but that 14 foreign staff remained in the country.

The SELEKA coalition brings together three former rebel groups that had largely been contained in CAR's northwest by government forces in recent years, but with foreign backing.

Paris in 2006 defended Bozize's government from a rebel advance using air strikes. President Francois Hollande on Thursday poured cold water on the latest request for help.

"Those days are over," he said.

Government soldiers were deployed at strategic sites and French troops reinforced security at the French Embassy after protesters threw rocks at the building on Wednesday.

With a government that holds little sway outside the capital, some parts of the country have long endured the consequences of conflicts spilling over from troubled neighbours Chad, Sudan and the Democratic Republic of Congo.

For a graphic on Central African Republic, click here: link.reuters.com/ret84t (Additional reporting by Aaron Maasho in Addis Ababa and by Tom Miles in Geneva; Writing by Richard Valdmanis; Editing by Michael Roddy)

• A Permanent Economic Emergency