Israeli Prime Minister Ehud Olmert expressed his "deep regret" to United Nations Secretary General Kofi Annan on Wednesday for the deaths of four U.N. observers in southern Lebanon after an Israeli airstrike on a U.N. post in the region Tuesday.

Four U.N. observers were killed when an Israeli airstrike struck their post the night before, Lebanese officials confirmed. Olmert promised a thorough investigation of the incident and said the results would be presented to Annan.

Ireland demanded Wednesday an explanation from Israel after the top-ranking Irish army officer in Lebanon complained that Israel ignored warnings that its forces were in danger of hitting United Nations observer posts.

The government summoned Ambassador Daniel Megiddo a day after the four were killed — within hours of six warning calls from Lt. Col. John Molloy, according to well-placed Irish sources.

Molloy, the top Irish officer in the United Nations' UNIFIL peacekeeping force in south Lebanon, spoke by phone Wednesday morning to Defense Minister Willie O'Dea and the commander of Ireland's army, Lt. Gen. Jim Sreenan.

Two well-placed sources, one military and one in the government, said Molloy complained he had called Israeli military liaison officers six times to point out that Israeli shellfire and aircraft munitions were landing dangerously close to more than one U.N. installation, including the one that suffered a direct hit Tuesday night.

Separately, an initial United Nations report Wednesday said its personnel made 10 warning calls in total before the attack. Reportedly, U.N. officials in New York and Lebanon repeatedly protested to Israel that a patrol base was coming under regular fire in the hours before an Israeli bomb leveled the building and killed at least three of the four unarmed military observers inside, a top peacekeeping official said Wednesday.

Lebanese officials, speaking on condition of anonymity because they were not authorized to speak to the press, said rescue workers were trying to extricate the fourth body from the wrecked building.

Annan said he was shocked by the "apparently deliberate targeting by Israeli Defense Forces of a U.N. observer post in southern Lebanon."

In response, Israel's U.N. Ambassador Dan Gillerman expressed his "deep regret" for the deaths, but denied Israel hit the post intentionally.

"I am shocked and deeply distressed by the hasty statement of the secretary-general, insinuating that Israel has deliberately targeted the U.N. post," he said, calling the assertions "premature and erroneous."

One of the dead was identified as Chinese U.N. observer Du Zhaoyu, China's official Xinhua News Agency reported. The Chinese Foreign Ministry said Israel's ambassador to Beijing was summoned Wednesday morning and asked to convey China's request that Israel fully investigate the incident and issue an apology to the victim's relatives.

"We are deeply shocked by this incident and strongly condemn it," Chinese Foreign Ministry spokesman Liu Jianchao said in the statement.

The other three UN observers were from Austria, Canada and Finland.

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The Associated Press contributed to this report.