Columns of foot soldiers have surrounded the presidential palace of Ivory Coast strongman Laurent Gbagbo following a day in which the United Nations and French forces opened fire with attack helicopters on the arsenal of the defiant ruler.

The fighters aiming to topple Gbagbo had succeeded in taking nearly the entire countryside in just three days last week, but faltered once they reached the country's largest city, where the presidential palace and residence are located.

With the help of the international forces, the armed group fighting to install the country's democratically elected leader, Alassane Ouattara, pushed their way to the heart of the city to reach Gbagbo's home late Monday.

They have surrounded it, and as of early Tuesday were waiting for him to step down, said a close adviser to Ouattara, who could not be named because he is not authorized to speak to the media.

Monday's offensive, which included air attacks on the ruler's home as well as three strategic military garrisons, marked an unprecedented escalation in the international community's efforts to oust Gbagbo, who lost the presidential election in November but has refused to cede power to Ouattara even as the world's largest cocoa producer teetered on the brink of all-out civil war.

Hundreds killed

The postelection violence has left hundreds dead — most of them Ouattara supporters — and has forced up to one million people to flee. Ouattara has used his considerable international clout to financially and diplomatically suffocate Gbagbo and pro-Ouattara forces before launching a dramatic military assault last week.

The office of French President Nicolas Sarkozy said Monday that UN Secretary General Ban Ki-moon had requested France's military participation in the day's attacks.

A UN helicopter fired on Gbagbo's troops at about 5 p.m. local time 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 told reporters in New York that Gbagbo's forces have consistently used heavy weapons against civilians and peacekeepers in recent days. He said the action was taken according to the mandate the mission has from the UN Security Council.

The especially strong resolution passed last week by the 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 UN 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.

Final battle

Earlier in the day, 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.

Explosions came from the city's downtown core, in the direction of the palace and a large military base. Machine-gunfire erupted on the lagoon-side 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 Ouattara had won the November presidential election, but Gbagbo refused to give up power after a decade in office.