Italy pledged a significant number of troops to the United Nations peacekeeping force in southern Lebanon today, stepping into the breach after France pledged a mere 400 troops instead of the thousands the UN had hoped for.

The composition of the urgently awaited force to police Monday's ceasefire between Israel and Hezbollah was today starting to emerge, after prolonged delays.

But the force's mandate has been heavily criticised by a former commander of UN blue berets in Bosnia, who warned that the peacekeepers would be powerless to stop a massacre if shooting broke out again, as their rules of engagement were so weak as to be a "recipe for disaster".

In the conflict zone, where a five-day ceasefire is still holding, the Lebanese army reached the country's southern border with Israel for the first time in 40 years - although it was largely a symbolic move.

A sole jeep, flying a large Lebanese flag and carrying just two soldiers in green camouflage uniforms, passed by the Fatima Gate in the village of Kfar Kila, best known as the place where the last Israeli forces withdrew from southern Lebanon in 2000 after an 18-year-occupation. Villagers threw rice and held Hezbollah banners, but the jeep did not stop.

Lebanon sent a first detachment of 2,500 troops across the Litani river into southern Lebanon yesterday, the first time it has been officially in control of the area since 1968, when it ceded authority to Palestinian guerrillas who used the mountanous region as a base for cross-border raids into Israel.

A total of 15,000 Lebanese troops are eventually supposed to deploy in the region under a UN Security Council resolution to end more than a month of fighting between Hezbollah guerrillas and Israel, although their deployment has so far been to predominantly Christian towns.

They are to be joined by an equal number of foreign troops, but potential donors are wary of being drawn into a no-win situation in which Hezbollah, the Shia Muslim guerrilla group, could again provoke Israel into battle.

France, the former colonial power in Lebanon, had widely been expected to lead the beefed-up UN force, but said yesterday that it would contribute only 400 soldiers. Germany, uneasy given its Nazi past at any possible confrontation with Israeli forces - has decided not to send a maritime force but no ground troops.

The Italian Government formally agreed this morning to send peackeeping troops to Lebanon. "It’s an important choice for the country, which is aware of its implications and consequences," said Romano Prodi, the Prime Minister, after a Cabinet meeting endorsed the move.

But Signor Prod did not give specifics, saying that the exact number of Italian troops to be sent to Lebanon and the details of the details of their deployment will be announced in the coming days when his Government has been given clear rules of engagement by the UN. Italy has said that it could sent as many as 3,000 soldiers to strengthen the existing force in southern Lebanon.

The UN still says that it hopes to have 3,500 troops on the ground within ten days and the entire 15,000-strong force by early November. But that timetable was questioned today by Lewis MacKenzie, a former Canadian general who headed the UN Protection Force in Bosnia during the early days of the siege of Sarajevo.

In an interview with BBC Radio 4's Today programme, General MacKenzie said that he expected it to take a year before the UN force was operational - and it would be hamstrung by a mandate, under Chapter 6 of the UN Charter rather than the more robust Chapter 7, that would allow it to use lethal force only in self-defence.

"We all understand that what comes out of the Security Council is the lowest common denominator for the best-case scenario. What will happen is the worst-case scenario and the UN will be ill-prepared to cope with it," he said.

"I understand that the UN commander will have to get approval from the Lebanese chain of command before he can use deadly force. I mean, it's a recipe for disaster."

General MacKenzie also warned that the lack of a proper mandate could see a repeat of the tragedies of Rwanda or Srebrenica, where UN commanders lacked the authority to prevent atrocities.

He said: "What will happen is that the UN commander, much like General Dallaire in Rwanda, will come back to the UN for permission to intervene and use force, and the UN will turn him down, and then the international community will condemn the commander for not doing what the international community thinks he should be doing.

"But if they read the fine print of the resolution it will be quite clear that he does not have does not have the authority to proceed."

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