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.