BEIRUT, July 25 - An Israeli air strike on Tuesday hit a U.N. base in southern Lebanon, killing four United Nations observers, Lebanese security sources said.

	Smoke rises from Khiam village after being hit by Israeli air strikes, in southern Lebanon, July 25, 2006. [Reuters]

The UNIFIL peacekeeping force in Lebanon confirmed that there were U.N. casualties but declined to say if any observers had been killed at the base, which appears to have collapsed while the U.N. observers were in the shelter.

"One aerial bomb directly impacted the building and shelter in the base of the United Nations Observer Group in Lebanon in the area of Khiam," said Milos Strugar, a UNIFIL spokesman.

"There are casualties among the observers. A UNIFIL dispatched rescue team which is on the spot is still unable to clear the rubble," he said.

"There were 14 other incidents of firing close to this position in the afternoon from the Israeli side and the firing continued during the rescue operation," he said.

In Jerusalem, an Israeli army spokeswoman said the military was investigating the report.

In Rome, U.N. Secretary-General Kofi Annan declined immediate comment on the bombing. "We are trying to get more details".

An Israeli tank shell hit a UNIFIL position in southern Lebanon on Monday, wounding four Ghanaian soldiers. Shrapnel from tank shells fired from the Israeli side seriously wounded an Indian soldier last week and Hizbollah fire wounded an Italian observer on the border on Sunday.

In 1996, during Israel's Grapes of Wrath campaign in Lebanon, an Israeli jet bombed a UNIFIL compound in the southern village of Qana, killing 106 civilians sheltering inside.

UNIFIL was created in 1978 after Israel's first major invasion of southern Lebanon and has been there ever since. The United Nations has called for a bigger, better armed, more robust international force in the area.