ISRAEL’S actions in Lebanon faced fresh international criticism last night after a United Nations base overlooking the border was hit by an Israeli bomb, killing up to four observers in its bunker.

A Lebanese official said the observers at the base were from China, Canada, Austria and Finland.

Kofi Annan, the UN Secretary-General, condemned the attack and called for an investigation. “I am shocked and deeply distressed by the apparently deliberate targeting by Israeli Defence Forces of a UN observer post in southern Lebanon,” Mr Annan said. “This co-ordinated artillery and aerial attack on a long established and clearly marked UN post at Khiam occurred despite per sonal assurances given to me by Prime Minister Ehud Olmert that UN positions would be spared Israeli fire,” he added.

Israel, denying that it targeted UN personnel, expressed regret over the “tragic deaths” of the observers and promised a thorough investigation.

The timing of the attack, just as Israel was trying to persuade Western powers to send troops for a beefed-up multinational border force, could not have been worse. “It certainly complicates the issue at a time when it is in Israel’s interests to have an international debate favourable to sending a multinational force,” a UN official said.

“I think it is going to make a lot of contributing countries very apprehensive about having their soldiers in a place where they are clearly facing an extremely dangerous situation from Israeli soldiers and Hezbollah fighters.”

The multinational force is at the centre of Western efforts to secure a ceasefire. Britain and the US hope that a peace plan involving a 10,000-strong force to act as a buffer Lebanon will be agreed in Rome today.

Diplomats believe that moderate Arab nations can be brought on board with a promise that Tony Blair and probably President Bush will demand a cessation of hostilities from both sides as soon as the force is agreed in principle.

The deal, backed by Mr Annan, will be presented as the “route to a ceasefire”, diplomatic sources said. Until now, Israel has had British and US support as it has resisted demands for an immediate unilateral ceasefire, prompting claims that it is buying time to complete its ferocious assault on Hezbollah militants.

Yesterday Israel said that the peacekeeping force should be as large as 20,000 and confirmed that it would take control of a “security zone” in southern Lebanon until the international peacekeepers arrive. “We have no other option,” said Amir Peretz, Israel’s Defence Minister. “We have to build a new security strip that will be a cover for our forces.”

As if to vindicate him, Sheikh Hassan Nasrallah, the leader of Hezbollah, went on television last night threatening to launch missiles deep into Israeli territory. “We are entering a new phase in the confrontation, the phase of (striking) beyond Haifa,” he said.

Saudi Arabia, Egypt and Jordan, the countries that Washington is relying on to rein in Hezbollah, made clear that they could not accept a US plan that would make a ceasefire only part of a broader peace deal.

In a rare public intervention, King Abdullah of Saudi Arabia called on world leaders to stop Israeli attacks into Lebanon, saying that if peace moves failed the Middle East could be engulfed in war.

Times Recommends

• The Japanese Prime Minister is doomed: or is he?
• Brighton company 'involved in cyber attack'
• The art of survival: it’s all in the mind