UNITED NATIONS — In a development that could prove a turning point in Israel’s war to rid its northern border of Hezbollah, the United Nations yesterday suffered casualties in the fighting, and Secretary-General Annan immediately accused the Israeli Defense Force of a “deliberate targeting” of four blue-helmeted U.N. observers.

Military sources said the incident occurred during an air and artillery attack near Khiyam, at the eastern region of southern Lebanon, where the IDF was preparing a large ground assault meant to create a Hezbollah-free buffer zone on the area north of Israel’s border. Four members of the United Nations’s interim force in Lebanon, identified as being from Canada, Austria, China, and Finland, were killed.

“I am shocked and deeply distressed by the apparently deliberate targeting by Israeli Defense Forces of a U.N. observer post in southern Lebanon,” Mr. Annan said yesterday in a late-night statement released in Rome, where he is attending a meeting of top players steering attempts to find solutions in the Lebanon war.

Citing no Israeli motivation for the alleged “deliberate” attack, Mr. Annan called on Israel to “conduct a full investigation.”

Israeli officials say they have an interest in avoiding targets that, if hit, would lead to widespread international pressure to end the campaign before Hezbollah’s military capabilities are significantly reduced.

Israel’s U.N. ambassador, Dan Gillerman, expressed “regret for the tragic incident,” adding, however, that he was “deeply distressed by the hasty statement of the secretary-general, insinuating that Israel has deliberately targeted the U.N. post Khiyam.”

In a statement released last night, Mr. Gillerman called Mr. Annan’s assertions “premature and erroneous.” While “demanding an investigation,” Mr. Annan “has already issued its conclusions,” Mr. Gillerman said.

Mr. Annan’s anger appeared to be personally directed at Prime Minister Olmert.

“This coordinated artillery and aerial attack on a long established and clearly marked U.N. post at Khiyam occurred despite personal assurances given to me by Prime Minister Ehud Olmert that U.N. positions would be spared Israeli fire,” his said in the statement.

“Furthermore,” Mr. Annan added, “General Alain Pelligrini, the U.N. Force Commander in south Lebanon, had been in repeated contact with Israeli officers throughout the day on Tuesday, stressing the need to protect that particular U.N. position from attack.”

Mr. Annan, who according to diplomats was accompanied by the U.N. peacekeeping chief, Undersecretary-General Jean Marie Guehenno, as he released the statement, believes he could not promote placing an international force in southern Lebanon if he does not appear to support his troops, according to a source familiar with yesterday’s statement.

Mr. Guehenno had a long dispute with Israel several years ago, when he had insisted that UNIFIL did not possess a videocassette depicting the Hezbollah kidnapping of Israeli soldiers. The cassette, videotaped by UNIFIL troops, eventually surfaced and the U.N. was forced to apologize and launch an internal investigation.

Israel has warmed up in recent days to the idea of deploying international troops in Lebanon once Hezbollah’s powers are degraded. According to Israel, the mandate of the force should be to assure that Hezbollah would not be rearmed, and to strengthen Lebanon’s army so it could eventually extend its control to the area now under full Hezbollah control.

So far, few countries have volunteered to join the force, and yesterday’s incident is expected to have a further chilling effect over potential troop contributors.

Mr. Annan, who has called for an “immediate cessation of hostilities” in Lebanon, wants the Rome summit of world leaders to lead to pressure on Israel to immediately end its Lebanon campaign. The top power at the Rome meeting, Secretary of State Rice, has steadfastly refused to pressure Israel to do so before Hezbollah terrorist and military abilities are reduced.

During a meeting with Mr. Olmert yesterday in Jerusalem, Ms. Rice “did not level any pressure on Israel to end the offensive against Hezbollah,” Israeli officials told the Jerusalem Post.

Before the two-hour meeting, Ms. Rice told reporters that the way forward depends on full implementation of Security Council resolution 1559, which calls for disarming all militias in Lebanon, including Hezbollah. The resolution’s implementation “will help to bring stability, and it will help to bring peace, and it will help a democratic Lebanon to fully emerge,” she said.

She spoke however prior to the UNIFIL incident, which could lead to further pressure on America to lean harder on Israel. America’s U.N. ambassador, John Bolton, declined to comment yesterday on Mr. Annan’s accusations. “It’s a very risky mission,” the French ambassador, Jean-Marc de la Sabliere, who serves as the rotating president of the Security Council, told reporters, referring to the peacekeepers.He added that all sides should “protect” UNIFIL.