BAMAKO, Mali — The international standoff with Islamists controlling northern Mali took a decisive turn Friday, as French forces engaged in an intense battle to beat back an aggressive militant push into the center of the country.

Responding to an urgent plea for help from the Malian government, French troops carried out airstrikes against Islamist fighters, blunting an advance by hundreds of heavily armed extremists, according to French officials and General Carter F. Ham, the top US military commander in Africa. One French helicopter was apparently downed, he said.