France has agreed to head an expanded UN peacekeeping force in Lebanon until February, the defence minister says.
The question of disarming Hezbollah has yet to be resolved
But the force must have a clear mandate and sufficient strength, Michele Alliot-Marie told French television.
Intense negotiations have been going on at the UN and in Beirut to build an advance force of up to 3,500 soldiers, to be deployed within two weeks.
Earlier Lebanon agreed to start moving its own 15,000-strong force south of the Litani River from Thursday.
The UN aim is to boost its limited existing force, Unifil, as soon as possible, enabling it to take over positions as Israel withdraws and the Lebanese troops move in.
UN PEACEKEEPING FORCE
Likely contributors: Italy, Turkey, Malaysia, Indonesia
Other possible contributors: Morocco, Spain, Belgium, Finland, Brunei, Germany, Portugal, Pakistan
Current Unifil force: 2,000 troops from China, France, Ghana, India, Ireland, Italy, Poland and Ukraine
The multinational force would then later be brought up to the full strength - 15,000 soldiers - set out in the UN ceasefire resolution passed on Friday.
But in agreeing to head the expanded force, Ms Alliot-Marie said it was vital to clearly define its mission.
"When you send in a force and its mission is not precise enough, and its resources are not well adapted or large enough, that can turn into a catastrophe, including for the soldiers that we send," she told France-2 television channel.
She did not say how many troops France would contribute.
The Lebanese army's southern deployment, meanwhile, looks set to end decades in which Hezbollah was in effect virtually unchallenged in the area.
Israel's military objectives during the month-long conflict included pushing Hezbollah out of a 30km (18-mile) wide "buffer zone" between the Israeli border and the Litani.
The UN ceasefire resolution calls for the area south of the Litani to be free of any "armed personnel, assets and weapons", except for the Lebanese and UN troops.
But the question of how and when Hezbollah will be disarmed or moved north - and by whom - has not been resolved.
Lebanese Prime Minister Fouad Siniora said the army would have complete control of the region, and no weapons would be allowed outside the authority of the state - but the BBC's Nick Childs in Beirut says precisely what this means for Hezbollah fighters remains ambiguous.
In New York, Israeli Foreign Minister Tzipi Livni accused Hezbollah of being in breach of the UN resolution, by failing to free two Israeli soldiers whose capture sparked the crisis.
"This is a moment of truth and maybe a test for the international community," she added.
"The resolution alone will do nothing unless we act with determination to ensure that this time the international community's decisions are totally implemented."
Ms Livni met UN Secretary General Kofi Annan to discuss implementing the resolution in full.
Mr Annan's deputy, Mark Malloch-Brown, said there was a real urgency in the situation.
"You have got a lot of trigger-happy people on both sides, a very high level of political distrust, a desire on both sides to make a point that they won and the other side lost, and therefore it is a tinder-dry environment where anybody can drop a spark and the flames go up again," he said.
"So the sooner we've got full disarmament south of the Litani and full deployment of the Lebanese and ourselves the better."
In other developments:
- A Lebanese general was ordered to be held for questioning after video images showed him drinking tea with Israelis who had taken over his barracks, AP news agency reported
- Israeli Defence Minister Amir Peretz has set up a commission to investigate the way the military campaign was conducted, amid strong criticism of the strategy from within Israel
- Rescue workers in the southern Lebanese town of Srifa recovered at least 30 more bodies from destroyed buildings, officials said.
In southern Lebanon, aid agencies are trying to deliver badly needed food and medicine, while the UN has warned returnees of the danger from unexploded ordnance.
The UN says about 250,000 people have already returned, and aid officials estimate that another 500,000 are on the move.
The BBC's Kim Ghattas in the southern town of Tyre says in some villages many homes have been destroyed and there is no electricity or running water.
Some returning families are heading back to Beirut after finding they have nothing to go back to, our correspondent reports.