A deadly exchange of fire between Israeli and Lebanese forces Tuesday shattered a tenuous calm that has persisted along the border since Israel and the Lebanese militia Hezbollah waged a month-long war four years ago that killed hundreds and displaced thousands.

In the most serious border confrontation since then, two Lebanese soldiers and one Lebanese journalist were killed. A 45-year old Israeli lieutenant colonel was also killed, and an Israeli platoon commander was critically wounded.

The United Nations said Wednesday that the Israeli troops were operating on their own side of the border when the shooting began.

The Israeli army said it opened fire in order to defend itself from Lebanese forces who shot at Israeli soldiers carrying out routine bush-clearing maintenance along the border. Lebanese media, by contrast, cited the Lebanese army as reporting that Lebanese soldiers fired warning shots at an Israeli patrol trying to cut down a tree and that the Israeli soldiers responded with gunfire.

"The trees being cut by the Israeli army are located south of the Blue Line on the Israeli side," Andrea Tenenti, deputy spokesperson for UNIFIL, said in a statement. Israel in 2000 withdrew all of its forces from south Lebanon, ending a 22-year occupation of the area.

Tuesday's clash raised the specter of a fresh conflict along Israel's border with Lebanon just as Israel faced an increase in rocket fire from the Gaza Strip, which is ruled by the Palestinian militant group Hamas.

What precisely happened along the Israel-Lebanon border remained somewhat murky late Tuesday. The U.N.'s peacekeeping force in Lebanon known as UNIFIL issued a statement Wednesday corroborating Israel's assertion that Israeli troops operated inside Israeli territory.

The U.N. peacekeeping force in Lebanon, known as UNIFIL, said it would investigate the circumstances that led to the incident, and U.N. Secretary General Ban Ki-moon said in a statement that he "urges the parties to exercise maximum restraint" to maintain calm. A senior U.S. official, speaking on condition of anonymity Tuesday because of the sensitivity of the diplomacy, said Lebanese forces appear to have started the incident. "We have touched base with all involved and based on what we know thus far, it appears the escalation started on the Lebanese side of the border and raises serious concerns," the source said.

The White House asked Defense Minister Ehud Barak to send President Obama's condolences to the family of the Israeli officer who was killed, Israel's Defense Ministry said Wednesday.

Tensions had been running high between Israel and Lebanon before the incident, with each side accusing the other of violating the terms of the United Nations cease-fire agreement that ended the 2006 war.

Israel has anxiously watched the Hezbollah militia, which pummeled Israeli towns with Katyusha rockets in 2006, rebuild an arsenal of tens of thousands of missiles of various range.

That buildup has led Israel to complain to the United Nations that its peacekeeping force, which was ramped up after the 2006 war, hasn't stopped the flow of war material to Hezbollah.

Israel says rockets, supplied mostly by Iran, are being trucked across the Syrian border into south Lebanon. The Lebanese government has complained to the United Nations about Israeli reconnaissance flights that violate Lebanese airspace and has accused Israel of trying to foment tensions in the region.

Amid these tensions, some experts have predicted that a new war could erupt soon.

"Although the border area between Israel and Lebanon is quieter than any time in the previous decade, speculation that a third Lebanon war will occur in the next twelve to eighteen months has been steadily rising," Daniel Kurtzer, a former U.S. ambassador to Israel and to Egypt, wrote in an analysis published by the Council on Foreign Relations last month.

Israeli Prime Minister Binyamin Netanyahu held the Lebanese government directly responsible for the border clash. In a statement issued by his office, Netanyahu said Israel "reacted and will react in the future" to any attempts to disrupt the quiet of its northern border.

That warning followed a threat by Barak last month that Israel would consider the Lebanese government directly responsible for any provocation by Hezbollah. An Iranian-backed Shiite militia that the United States designates as a terrorist group, Hezbollah is also a powerful member of the ruling Lebanese coalition.

Lebanese Prime Minister Saad Hariri condemned what he called Israel's violation of Lebanese sovereignty on Tuesday and demanded that the "international community bear their responsibilities and pressure Israel to stop its aggression.''

An Israeli army spokeswoman, Avital Leibovich, said the encounter started when Israeli troops were "clearing bushes for operational reasons'' in Israeli territory along the border.

The internationally recognized border between Israel and Lebanon is delineated by barrels and other markers in the area where the clashes took place. In addition, there is an Israeli security fence along the border that dips in some places into Israeli territory.

The Israeli army said it had notified U.N. peacekeeping forces of its bush-clearing plans earlier in Tuesday. As the army began to work in mid-afternoon, Leibovich said, Lebanese soldiers shouted to the Israeli soldiers to leave the area and Israeli soldiers responded that they were operating within Israeli territory.

She said Lebanese soldiers then opened fire at the Israeli commanders and Israel retaliated with artillery and small arms.

"We were asked to hold our fire so the Lebanese could evacuate'' their wounded. "Half an hour later we were attacked by a [rocket-propelled grenade]. They directed fire towards an Israeli tank. Then we retaliated with an Israeli helicopter,'' Leibovich added.

Leibovich said the Israeli army believed the attack was preplanned.

The 2006 war erupted after Hezbollah fired rockets at Israeli towns and launched a deadly ambush on an Israeli border patrol. Israel responded by firing at targets inside Lebanon and for 34 days mortars and missiles rained down on both sides of the border.

Some commentators in Israel perceived Tuesday's incident as an attempt by Hezbollah to reassert itself ahead of a forthcoming indictment by an international panel investigating the 2005 murder of Prime Minister Rafiq al-Hariri, the current Lebanese leader's father.

"Hezbollah has to prove they are required for defending the south," said former Israeli deputy defense minister Ephraim Sneh. "They create a provocation to show that they are still needed."

Leaders from Saudi Arabia to Syria have flocked to Lebanon in recent days to urge calm amid reports that the U.N. investigator is likely to indict members of Hezbollah in Hariri's killing. A U.N. diplomat said charges are likely to be announced this fall.