A United Nations helicopter fired at strongman Laurent Gbagbo's forces on Monday as France authorized its military to take out his heavy weapons, an unprecedented escalation in the international community's efforts to oust the entrenched leader.

The office of French President Nicolas Sarkozy said U.N. Secretary-General Ban Ki-moon had requested France's military participation. Gbagbo lost presidential elections in November but has refused to cede power to Alassane Ouattara even as his West African nation teetered on the brink of all-out civil war.

The two men have vied for the presidency for months, with Ouattara using his considerable international clout to financially and diplomatically suffocate Gbagbo. Forces backing Ouattara launched a dramatic offensive last week, seizing control of the administrative capital and other towns before heading toward Abidjan.

On Monday, the U.N. helicopter fired on Gbagbo's troops at about 5 p.m. local time (1700 GMT) to prevent them from using heavy weapons at the Akouedo camp in Abidjan, said Nick Birnback, the spokesman for the U.N. Department of Peacekeeping Operations.

"The country has been plunged into violence with a heavy toll on the civilian population," Ban said in a statement released Monday. "In the past few days, forces loyal to Mr. Gbagbo have intensified and escalated their use of heavy weapons such as mortars, rocket-propelled grenades and heavy machine guns against the civilian population in Abidjan."

An especially strongly worded resolution passed last week by the U.N. Security Council condemned "in the strongest terms the recent escalation of violence throughout the country which could amount to crimes against humanity."

The unanimous resolution also stressed the council's "full support" for the U.N. peacekeeping force in Ivory Coast "to use all necessary means to carry out its mandate to protect civilians under imminent threat of physical violence ... including to prevent the use of heavy weapons against the civilian population."

Frederic Daguillon, the spokesman for the French force Licorne protecting civilians in Ivory Coast said earlier Monday on France-Info radio that the total French military presence in the former French colony is 1,650.

Meanwhile, fighters backing Ouattara entered Abidjan by the truckload Monday afternoon as part of a final offensive to take the last piece of the West African country still largely controlled by Gbagbo.

Residents in two different districts in northern Abidjan reported seeing soldiers advancing into the city. Thousands of troops had been amassing outside Ivory Coast's commercial capital since last week, readying for the final battle to topple Gbagbo and install Ouattara.

Alain Lobognon, a spokesman for Ouattara's defense minister, confirmed by telephone that the general offensive had begun Monday afternoon.

Their target is the presidential palace and the mansion where Gbagbo is believed to be holed up. Both are located on the edges of a lagoon in the heart of the country's biggest city.

Explosions came from the city's downtown core, in the direction of the palace and a large military base. Machine gunfire erupted on the lagoonside highway just two blocks from the palace.

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Associated Press writers Jenny Barchfield in Paris and Anita Snow at U.N. headquarters in New York contributed to this report.