"ABIDJAN, Ivory Coast – The United Nations and French forces opened fire with attack helicopters Monday on the arsenal of this country's entrenched ruler, as columns of Ivorian fighters allied with his challenger finally pierced the city limit.

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

Monday's offensive marked an unprecedented escalation in the international community's efforts to oust Gbagbo, who lost the presidential election in November yet has refused to cede power to Alassane Ouattara even as the world's largest cocoa producer teetered on the brink of all-out civil war.

The postelection violence has left hundreds dead — most of them Ouattara supporters — and has forced up to 1 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.

On Monday, the U.N. fired on the Akouedo military base at around 5 p.m. local time (1700 GMT) to prevent Gbagbo's forces from using heavy weapons against civilians, said the spokesman for the U.N. Department of Peacekeeping Operations Nick Birnback.

Explosions resonated from the city's downtown core a few blocks from the presidential palace and near the base of the republican guard, and those living nearby barricaded their windows with mattresses. Flames could be seen licking the sky above the home of the staunchly pro-Gbagbo republican guard.

French President Nicolas Sarkozy said in a statement that he had authorized the 1,600-strong French Licorne force based here to help in the operation following an appeal from U.N. Secretary-General Ban Ki-moon, who said that the use of force was necessary to prevent further attacks on civilians..."

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