"France will stay for as long as it takes to bring stability to Afghanistan," Fillon told a joint press conference with Hamid Karzai after meeting the Afghan President.
On Monday France's Foreign Affairs Minister Bernard Kouchner announced that contact has been established with the kidnappers of two journalists from the France 3 TV channel. Fillon declined to give more details but said that the government is working with the Afghan government to obtain their freedom.
France has agreed to send 80 new instructors to Afghanistan, far fewer than Washington hoped for in the aftermath of the London conference on Afghanistan.
"We have committed the necessary means to accomplish the mission that has been entrusted to us," Fillon told the media on Thursday.
Before meeting Karzai, Fillon attended a ceremony at Kabul airport to mark the repatriation of the body of a 20-year-old soldier killed two days ago. He was the 40th French death since the 2001 invasion.
Fillon, who is accompanied by French army Chief of Staff Jean-Louis Georgelin, is to meet Stanley McChrystal, the US officer in charge of the Afghan international military mission.
Also on Thursday, the Taliban vowed it would use hit-and-run tactics to fight a pro-government offensive due to start within the next few days.
US Marines and Nato and Afghan government troops have massed outside the Taliban bastion of Marjah in Helmand province ready for what has been billed as one of the biggest operations since 2001.