UN peacekeepers in south Lebanon contacted Israeli troops 10 times before an Israeli bomb killed four of them, an initial UN report says.
The post was hit by a precision-guided missile after six hours of shelling, diplomats familiar with the probe say.
UN-led crisis talks in Rome ended with no agreement to urge an immediate ceasefire between Israel and Hezbollah.
Eight Israeli soldiers have died in ongoing clashes - the biggest loss in one incident since the conflict began.
Twenty-two soldiers were injured as Israeli troops tried to gain control of the town of Bint Jbeil, a strategically located Hezbollah stronghold.
An Israeli officer died in a separate clash later.
And a massive explosion destroyed a several-storey building in the centre of Tyre housing the offices of a top Hezbollah commander.
He was not there at the time.
A senior Israel army general said he expected the fighting would continue for "several more weeks".
More than 400 Lebanese and 42 Israelis have died in two weeks of conflict, which began after Hezbollah captured two Israeli soldiers in a cross-border raid on 12 July.
In other developments:
- Hezbollah fired more than 100 rockets into Israel, injuring 31 people, security and medical sources say
- A Jordanian military plane arrived in Beirut to evacuate some of the most seriously wounded Lebanese civilians
- Ten lorries loaded with food and medical supplies arrived in the southern town of Tyre from the capital, Beirut
- Hezbollah's leader Hassan Nasrallah warned on TV that his organisation would begin firing rockets further south into Israel than Haifa
- More than 300 people - mainly US and Australian citizens - who had been caught in the fighting in southern Lebanon are due to leave from Tyre on a Canadian ferry on Wednesday evening
The four unarmed UN observers from Austria, Canada, China and Finland, died after their UN post in the town of Khiam was hit by an Israeli air strike on Tuesday.
The UN report says each time the UN contacted Israeli forces, they were assured the firing would stop.
A senior Irish soldier working for the UN forces had warned the Israelis six times that their bombardment was endangering the lives of UN staff, Ireland's foreign ministry said.
Had Israel responded to the requests, "rather than deliberately ignoring them", the observers would still be alive, a diplomat familiar with the report said.
Israeli Prime Minister Ehud Olmert has expressed "deep regrets" over the deaths.
Israel is conducting an investigation into the incident.
It has rejected accusations made by UN Secretary General Kofi Annan that the targeting of the UN position was "apparently deliberate".
White House spokesman Tony Snow said "something went really wrong" to cause the deaths, but also said there was no reason to suggest the bombing was deliberate.
The UN Security Council is meeting to discuss the incident.
The Rome summit, called by US Secretary of State Condoleezza Rice, brought together EU and Arab nations plus the US and Russia, but not Israel, Iran or Syria.
The conference released a declaration expressing "determination to work immediately to reach with utmost urgency a ceasefire to put an end to the current hostilities".
It also said a ceasefire "must be lasting, permanent and sustainable".
The statement called for an international force with a UN mandate for south Lebanon, and the full implementation of existing UN Security Council resolutions calling for the disarming of militias and deployment of Lebanese troops in the border region.
Mr Annan said it was important to work with the countries of the region, including Syria and Iran, to find a solution to the crisis.
But Condoleezza Rice was critical of the role of both countries.
"It's not a question of talking to Syria, it's whether Syria's prepared to act," she said.
In an impassioned speech, Lebanese Prime Minister Fouad Siniora warned that more people would die if the ceasefire was delayed, and called for a Lebanese-Israeli prisoner exchange as part of plan to end the fighting.