President Jacques Chirac has pledged an additional 1,600 French troops to the UN peacekeeping force in south Lebanon.
Chirac said concerns about the force's mandate had been met
He announced the despatch of a further two battalions in a televised address in which he also said France was willing to continue leading the force.
He described the ceasefire between Israeli forces and Hezbollah guerrillas as "fragile".
France was heavily criticised over its initial offer of just 200 extra troops for the UN force in its former colony.
Along with the 200 troops who were already in the long-established UN Interim Force in Lebanon (Unifil), this latest move will bring the total French contingent in Lebanon to 2,000.
Before Mr Chirac's announcement, Italy, which has pledged 3,000 troops, had offered to lead the augmented force.
Pressure has grown for the expanded UN force - which should eventually include 15,000 troops - to be quickly deployed to southern Lebanon to monitor the fragile ceasefire, now in its 11th day.
The UN had been disappointed by the response of European nations, which have expressed concern about the force's mandate, particularly on the issue of disarming Hezbollah guerrillas.
And Syria has threatened to close its frontier with Lebanon if UN peace-keepers are stationed there - a move that led to Israel accusing it of undermining the ceasefire.
'Very good sign'
Mr Chirac said he had received the necessary assurances from the UN about the force's mandate and chain of command.
His announcement was welcomed by the UN and US President George W Bush.
COST OF CONFLICT
About 1,000 - mostly civilians
No precise data on Hezbollah dead
700,000 - 900,000 (UNHCR; Lebanese govt)
500,000 (Human Rights Watch)
Mr Chirac said: "Two extra battalions will deployed on the ground to extend our numbers within Unifil.
"Two thousand French troops will therefore be placed under the control of the United Nations in Lebanon.
"France is ready, if the United Nations wishes, to continue commanding this force."
A special meeting of EU foreign ministers to discuss Lebanon, which will be attended by UN Secretary General Kofi Annan, will be held in Brussels on Friday.
A UN spokesman said: "We very much welcome the French offer. We look forward to tomorrow's meeting with the Europeans, where we hope more offers will be forthcoming.
"This is a very good sign of the commitment the international community is willing to make to the force. This is extremely positive."
In a statement President Bush described the French move as "an important step" and said he applauded the decision.
The ceasefire was brokered by the UN after weeks of fighting in southern Lebanon. The conflict was sparked by a cross-border raid by Hezbollah fighters in which they captured two Israeli soldiers and killed eight others.
Throughout the military campaign against Lebanon, Israel's twin aims were the return of the captured Israeli soldiers and the removal of Hezbollah's influence from southern Lebanon.
The Israeli army lost 116 soldiers. Forty-three civilians were also killed by more than 4,000 Hezbollah rocket attacks.
Halutz has been widely criticised for not achieving Israel's war aims
About 1,200 Lebanese were killed in the conflict, mostly civilians in Israel's vast bombardment of the county and land invasion in the south.
Relief workers are now able to reach most of those areas of southern Lebanon that were cut off during the conflict.
Most of the one million people who fled the area have now returned, but many still have no proper access to basic utilities, UN emergency relief co-ordinator Jan Egeland said.
"Food is in general less of a problem than water and sanitation. We also have probably hundreds of thousands who come back and see that their housing is either destroyed or badly damaged," he told the BBC.
Meanwhile, Israeli military chief of staff Lt Gen Dan Halutz has publicly admitted to failings in the conflict with Hezbollah militants in Lebanon.
In a letter to troops, he said it had exposed shortcomings in the military's logistics, operations and command.
There would be a thorough and honest investigation, he promised.