The Israeli Prime Minister, Ehud Olmert, spoke by phone to Mr Annan. Mr Olmert expressed his "deep regret" and said the peacekeepers were killed mistakenly. He expressed dismay over Mr Annan's accusation and promised a thorough investigation , saying the results would be presented to Mr Annan.
Mr Annan later called for participants at a Mideast conference to push for an immediate ceasefire to end fighting between Israel and Hizbollah guerrillas.
Hizbollah must stop its "deliberate targeting of Israeli population centers". And Israel must put an end to all bombing, ground operations and blockades of Lebanese ports.
In a statement in Rome, Mr Annan said: "A key stipulation for such a halt in fighting would be that the parties must not, I repeat, must not take advantage of such a pause to conduct offense operations, redeploy or resupply." And he added that an international force will be vital to keeping peace.
Since fighting between Israel and Hezbollah militants began two weeks ago, there had been several dozen incidents of firing close to UN peacekeepers and observers, including direct hits on nine positions, some of them repeatedly, a UN official said.
As a result of these attacks, 12 UN personnel have been killed or injured, the official said.
Last night's bomb made a direct hit on the building and shelter of the observer post in the town of Khiam, near the eastern end of the border with Israel, said Milos Struger, spokesman for the UN peacekeeping force in Lebanon known as Unifil.
Four unarmed military observers were in a bunker and the bunker collapsed as a result of the bombing, the UN official said.
Rescue workers were trying to clear the rubble, but Israeli firing " continued even during the rescue operation", Struger said.
Annan said two UN military observers were killed with two more feared dead. Later, the UN official confirmed that a third body was recovered from the rubble.
The victims included observers from Austria, Canada, China and Finland, UN and Lebanese military officials said.
Chinese foreign minister Li Zhaoxing said he was saddened by the news and that it showed "we should try harder to call on the parties to be restrained and to be calm and restore the peace process of the Middle East immediately".
China's official Xinhua News Agency identified the Chinese victim as Du Zhaoyu.
It was not immediately known which of the others were confirmed dead.
Annan said the "co-ordinated artillery and aerial attack on a long-established and clearly marked UN post at Khiam occurred despite personal assurances given to me by prime minister Ehud Olmert that UN positions would be spared Israeli fire."
Furthermore, he said, General Alain Pelligrini, the UN force commander in south Lebanon, had been in repeated contact with Israeli officers throughout yesterday "stressing the need to protect that particular UN position from attack".
But Gillerman said he was "shocked and deeply distressed by the hasty statement of the secretary general, insinuating that Israel has deliberately targeted the UN post", calling the assertions "premature and erroneous".
He said Olmert's assurances to the secretary-general were "a clear indication" of Israel's commitment to ensure the safety and security of UN personnel.
As reports of the attack emerged, Annan rushed out of a hotel in Rome following a dinner with US secretary of state Condoleezza Rice and Lebanese prime minister Fuad Saniora.
"I am shocked and deeply distressed by the apparently deliberate targeting by Israeli Defence Forces of a UN observer post in southern Lebanon," Annan said in a statement later.
"I call on the government of Israel to conduct a full investigation into this very disturbing incident and demand that any further attack on UN positions and personnel must stop."
Gillerman said "Israel is carrying out a thorough inquiry into this tragic incident and will inform the UN of its results as soon as possible".
The UN Security Council is expected to receive a briefing on the bombing today.
Since Israel launched a massive military offensive against Lebanon and Hezbollah guerillas on July 12, a Nigerian civilian employee working with Unifil and his wife had been killed in the crossfire in the southern port city of Tyre.
Five Unifil soldiers and one military observer had also been wounded, Struger said. They included four Ghanaians injured by artillery fire on Monday and a peacekeeper shot through the back on July 23.
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