Abidjan: The military forces of United Nations fired at President Laurent Gbagbo's forces as France authorised its military forces to take out his heavy weapons.

The office of French President Nicolas Sarkozy said UN Secretary-General Ban Ki-moon had requested France's military participation.

Gbagbo lost presidential elections in November but refused to cede power even as his nation teetered on the brink of all-out civil war.

The helicopter fired on Gbagbo's troops at about 5 pm local time (2230 IST) to prevent them from using heavy weapons at the Akouedo camp in Abidjan, said the spokesman for the UN Department of Peacekeeping Operations.

Spokesman Nick Birnback said in New York that Gbagbo's forces have consistently used heavy weapons against civilians and peacekeepers in recent days.

The attacks on Gbagbo positions marked a dramatic escalation in the offensive to oust the strongman who has refused to cede power for more than four months since he was declared the loser of the presidential election.

Frederic Daguillon, the spokesman for the French force Licorne protecting civilians in Ivory Coast said that the total French military presence in the former French colony is 1,650.

Meanwhile, fighters backing democratically elected leader Alassane Ouattara entered Abidjan by the truckload 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 this 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.

(Agencies)