United Nations chief Kofi Annan confirmed the commitment of 7,000 European troops after talks with EU ministers in Brussels.
There are currently 2,000 UN troops on the ground in Lebanon.
Mr Annan says the EU troops will create the backbone of a credible force which should start arriving within days.
"We may have a unique opportunity to transform the cessation of hostilities into a durable ceasefire," Mr Annan said in Brussels.
Italian Prime Minister Romano Prodi says the Italian contingent could leave as soon as Tuesday.
The Italian deployment will depend on approval by cabinet at meeting on Monday.
But EU officials warn it could be months before the full force is deployed.
Israel has reiterated that it will not finally withdraw from southern Lebanon until the UN peacekeeping force is deployed.
France takes command
Mr Annan said the UN force will led by France which has offered to bolster its troops to 2,000, after initially offering just 200 troops.
UN officials want the expanded UN Interim Force in Lebanon (UNIFIL) to have a strong European contingent.
It will support 15,000 Lebanese troops deployed to the south to shore up the fragile ceasefire between Israel and Hezbollah.
The expanded force is crucial to the implementation of UN Resolution 1701 which brought an end to the 34-day conflict.
The EU contribution will include up to 6,900 soldiers, as well as air and naval assets, said Finnish Foreign Minister, Erkki Tuomioja.
Finland currently holds the rotating EU presidency.
Italy to take over from France
The UN chief has asked Italy to take over command of the force from France when its term expires at the end of January.
Italy is expected to supply up to 3,000 soldiers, which would make it the biggest contributor to the force.
There will also be a strategic cell created to help provide military guidance to the troops on the ground.
It will be based at UN headquarters in New York and led by an Italian general.
French President Jacques Chirac earlier said the full force of 15,000 UN troops was "excessive" and made "no sense."
France, though, played a key role diplomatically in revamping the force, having co-authored the UN resolution with the United States.
On Friday, a contingent of 170 French soldiers arrived at the Lebanese port of Naqoura, bringing specialists in demining and reconstruction.
However, the French general currently in charge of UNIFIL, Alain Pellegrini, told French radio channel RTL that he for one had not received "clarifications" about the enlarged force's role.
"I don't know what clarifications the president has received," he said.
French Defence Minister Michele Alliot-Marie said later the division of labour between UNIFIL's parts would be worked out in the next few days.
"What is important for France is that there be only one leader on the ground", in order to avoid any possible "contradiction" between a UN civilian leadership and the military force.
"This is one of the things which we have secured from the UN", she said.
Details of EU contributions
The announcement of the French reinforcements spurred some of the other EU countries into firming up previously vague promises to contribute.
Spain was said to be ready to send 1,000 to 1,200 soldiers, substantially more than the 800 initially cited in media reports.
Polish Foreign Minister Anna Fotyga said that Poland would boost its contribution by 300 to around 500.
Belgian Prime Minister Guy Verhofstadt said his country would provide 300 soldiers by late September or early October, with the possibility of adding nearly 100 more later.
Finland pledged 250 soldiers.
Britain's Europe Minister Geoff Hoon said that while his country's military -- heavily deployed in Iraq and Afghanistan -- was too stretched to send ground troops, it might supply "specialised" units.
Greece is offering a frigate, helicopter and special forces while other countries, notably Portugal and Latvia, have indicated they could send soldiers, but no details were given Friday.
Germany, wary of potential confrontations with Israel because of its Nazi past, is prepared to offer naval units, not troops, as is Sweden.
Russia, which is not an EU member, said Friday it was still considering the possibility of sending troops.
Mr Annan said Bangladesh, Malaysia and Indonesia had offered to participate in UNIFIL -- though Israel has opposed contributions from those Muslim countries because of an absence of diplomatic relations.
The force is also to help Lebanon halt arms smuggling to Hezbollah over the border with Syria, considered a main sponsor of the Lebanese militia, if Beirut requests.
Syria is concerned at the deployment of foreign troops in Lebanon – from which it was forced to withdraw its soldiers last year under UN pressure led by France -- and has warned it may close the border.