UN officials said Hezbollah militants had been operating in the area of the post near the eastern end of the border with Israel, a routine tactic to prevent Israel from attacking them.

A Lebanese man reacts as he looks at destruction caused by Israeli air strikes in the southern suburbs of Beirut, a strong hold of Hezbollah, July 25, 2006. [AFP]

"We did repeatedly in recent days say (to Israel) that this was an exposed position, that Hezbollah militants were 500 meters (yards) away shielding themselves near UN workers and civilians," UN humanitarian chief Jan Egeland said. "That's why it is so inexplicable that what happened."

Israeli officials had told the United Nations that the bombing around the base was part of an "an aerial preparation for a ground operation," said the senior official, who spoke on condition of anonymity.

Officials in the outpost called the Israeli army 10 times during those six hours, and each time an army official promised to have the bombing stopped, according to a preliminary UN report on the incident.

Once it became clear those pleas were being ignored, the force's commander sought the involvement of top officials in New York, a senior UN official in New York said, speaking on condition of anonymity because the investigation of the incident was not yet complete.

UN Deputy Secretary-General Mark Malloch Brown and Lute herself then made several calls to Israel's UN mission "reiterating these protests and calling for an abatement of the shelling," Lute said.

UN officials said the observation position was well marked. A picture the world body released Wednesday showed the three-story building was painted white with the letters "UN" emblazoned in large black letters on all sides, and a light blue UN flag hung from a nearby flagpole that was roughly 50 feet high.

Witnesses said the building, which was surrounded by concrete blast walls and barbed wire, also had the letters UN painted on the roof and it was illuminated by floodlights at night.

During the shelling, the observers took refuge in a bomb shelter designed to withstand a strike by a 155mm artillery shell, UN officials said. The bunker collapsed in the attack, and the extent of the damage suggests it was hit with a large bomb, said Brig. Gen. J.P. Nehra, the deputy force commander for the UN peacekeeping force in Lebanon known as UNIFIL.

Since fighting between Israel and Hezbollah militants began two weeks ago, there have 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, UN officials said.