A United Nations helicopter has fired at Ivory Coast president Laurent Gbagbo's forces as France authorised its military forces to take out his heavy weapons.
UN secretary-general Ban Ki-moon requested France's military participation, according to the office of French president Nicolas Sarkozy.
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 5pm local time on Monday (6pm UK time) to prevent them from using heavy weapons against the Akouedo camp in Abidjan, the UN Department of Peacekeeping Operations said, adding that Gbagbo's forces have consistently used heavy weapons against civilians and peacekeepers in recent days. The action was taken according to the mandate the mission has from the UN Security Council.
The attacks on Gbagbo positions marked a dramatic escalation in the offensive to oust him since he was declared the loser of the presidential election. The total French military presence in the former French colony is 1,650.
Meanwhile, fighters backing democratically-elected leader Alassane Ouattara entered the city of Abidjan by the truckload on Monday 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 massing outside Ivory Coast's commercial capital since last week, readying for the final battle to topple Gbagbo and install Mr Ouattara. 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 centre, in the direction of the palace and a large military base. Machine gunfire erupted on the lagoonside highway just two blocks from the palace.
International observers and governments around the world backed the results issued by Ivory Coast's electoral commission, showing Mr Ouattara had won the November presidential election, but Gbagbo refused to give up power after a decade in office.
The two men have vied for the presidency for months, with Mr Ouattara using his considerable international clout to suffocate Gbagbo financially and diplomatically.