President Nicolas Sarkozy has arrived in Afghanistan to support French troops a day after one of the deadliest attacks on France's forces abroad, BBC reported.
Ten French soldiers were killed and 21 injured in an ambush by Taleban fighters east of the capital, Kabul.
Mr Sarkozy said France was committed to the fight against terrorism, and the mission in Afghanistan would continue.
France plans to send in 700 troops by the end of August, bringing its presence in Afghanistan to 2,600.
The French news agency AFP reported that Tuesday's deaths brought to 24 the number of French troops killed in Afghanistan since 2002.
The BBC's Emma-Jane Kirby, in Paris, says news of the deaths will provoke anger in France, where two-thirds of people say they are opposed to French involvement in the conflict.
The loss of life is thought to be the heaviest suffered by the French military since 58 paratroopers were killed in Beirut in 1983.
Mr Sarkozy is expected to meet some of the French soldiers injured in the ambush, the AFP news agency reports, as well as Afghan President Hamid Karzai.
He arrived in Kabul early on Wednesday, accompanied by French Foreign Minister Bernard Kouchner and Defence Minister Herve Morin, AFP said.
Before leaving France, the president said he was travelling to Kabul to assure French troops "that France is at their sides".
In a statement, he said France had "been struck severely" but that France was "resolved to pursue the fight against terrorism, for democracy and liberty".
Mr Sarkozy said: "The cause is just, it is the honour of France and its armies to defend it."
The French troops were caught up in fighting that started on Monday in the area of Sarobi, some 50km (30 miles) from Kabul.
French defence officials said about 100 soldiers - from France, the US and Afghanistan - were on a reconnaissance mission when bad road conditions forced them to stop their vehicles.
A group of French soldiers was sent ahead on foot to check the terrain, but they were ambushed by Taleban fighters and nine were killed.
A 10th French soldier was killed when his vehicle overturned on the road.
An Afghan intelligence officer told the BBC the troops were ambushed from several directions by heavily-armed Taleban and al-Qaeda forces.
The fighting went on for 24 hours and it is understood that reinforcements had to be called in to airlift the troops to safety.
The deaths came amid warnings that insurgents are closing in on the capital city.
The French recently took over control of the Kabul regional command, which includes Sarobi.