AXIM, Ghana — The United Nations and France went on the offensive Monday against Ivory Coast’s strongman, Laurent Gbagbo, striking targets at his residence, his offices and two of his military bases in a significant escalation of the international intervention into the political crisis engulfing the nation.

France, which showed a newfound muscularity by championing military strikes against Col. Muammar el-Qaddafi’s forces in Libya, attacked heavy artillery and armored vehicles at Mr. Gbagbo’s residence and presidential offices, two centers of his power, a French military spokesman said Monday night.

The United Nations said it had also launched helicopter strikes against Mr. Gbagbo’s forces at two of his bases, to prevent them from using the kinds of heavy weapons that have been aimed at civilians and United Nations personnel during the crisis.

By early Tuesday, Mr. Gbagbo was in a bunker beneath his residence and was negotiating a possible surrender through the French ambassador, according to Alain Lobognon, a spokesman for the prime minister, Guillaume Soro. Forces supporting Mr. Gbagbo’s rival, Alassane Ouattara, were several hundred feet away.