An Israeli bomb destroyed a U.N. observer post on the border in southern Lebanon Tuesday, killing three observers and leaving another feared dead, officials said. U.N. chief Kofi Annan said Israel appeared to have struck the site deliberately.

Israeli Prime Minister Ehud Olmert on Wednesday expressed "deep regret" over the killings and said the peacekeepers were killed mistakenly. He spoke on the phone with Annan, expressing dismay over Annan's accusation and promising a thorough investigation, according to Olmert's office.

The bomb made a direct hit on the building and shelter of the observer post in the town of Khiam near the eastern end of the border with Israel, said Milos Struger, spokesman for the U.N. peacekeeping force in Lebanon known as UNIFIL.

Annan said two observers were killed with two more feared dead. Later, a U.N. official confirmed that a third body had been recovered. The official spoke on condition of anonymity because he was not authorized to publicly discuss the issue.

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 observers were from Austria, Canada and Finland but it wasn't clear which two were confirmed killed, U.N. and Lebanese military officials said.

• CountryWatch: Israel | Lebanon | Syria | Iran

Israel's U.N. Ambassador Dan Gillerman expressed his "deep regret" for the deaths and denied the post was intentionally targeted.

Rescue workers were trying to clear the rubble, but Israeli firing "continued even during the rescue operation," Struger said.

U.N. officials said four observers were in the post when the bomb hit, and the building had been destroyed. Two bodies had been recovered and two were unaccounted for, apparently still in the rubble. They spoke on condition of anonymity because of the sensitivity of the issue.

The victims included observers from Austria, a Canada, China and Finland, U.N. and Lebanese military officials said, speaking on condition of anonymity because they were not authorized to release the information to the media. It was not immediately known which were confirmed dead.

As reports of the attack emerged, Annan rushed out of a hotel in Rome following a dinner with U.S. Secretary of State Condoleezza Rice and Lebanese Prime Minister Fuad Saniora.

"I am shocked and deeply distressed by the apparently deliberate targeting by Israeli Defense Forces of a U.N. Observer post in southern Lebanon," Annan said in the statement.

Annan said in his statement that the post had been there for a long time and was marked clearly, and was hit despite assurances from Olmert that U.N. positions would not be attacked.

"I call on the government of Israel to conduct a full investigation into this very disturbing incident and demand that any further attack on U.N. positions and personnel must stop," Annan said in the statement.

Gillerman called the assertions "premature and erroneous."

"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.

He said Israel would investigate the bombing. "We do not have yet information what caused this death: it could be the IDF (Israel's military) it could be Hezbollah," he said.

In the meantime, the envoy assured that "Israel remains committed to protecting the safety and security of U.N. personnel on the ground and is doing its utmost to guarantee that they be able to carry out their mission."

The U.N. Security Council was expected to receive a briefing on the bombing on Wednesday.

Since Israel launched a massive military offensive against Lebanon and Hezbollah guerrillas July 12, an international civilian employee working with UNIFIL and his wife have been killed in the crossfire between Israeli forces and Hezbollah guerrillas in the southern port city of Tyre.

Five UNIFIL soldiers and one military observer have also been wounded, Struger said.