Lebanese troops traded fire with Israeli forces across the border for the first time since last summer's war between Hezbollah and Israel, showing how tense the boundary remains nearly six months after a U.N.-brokered truce.

U.N. peacekeepers in south Lebanon described the shootout late Wednesday as a "serious incident," though it was not expected to undermine the truce that ended 34 days of fighting.

Peacekeepers contacted both sides, "urging them to cease hostilities with immediate effect," said Liam McDowell, a spokesman for the force.

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Lebanese officials said their troops fired on an Israeli army bulldozer that had crossed the frontier near the border village of Maroun el-Rass, which saw heavy fighting in the summer.

The vehicle crossed the so-called Blue Line — the U.N.-demarcated boundary — and ventured about 20 yards into Lebanon, Lebanese military officials said.

The officials, speaking on condition of anonymity pending the release of a formal army statement, said the army fired machine guns at the bulldozer. Israeli forces responded with five anti-tank grenades that targeted a Lebanese armored vehicle and transport jeep, the officials said.

There were no reports of injury on either side.

Israel confirmed the exchange — with security officials saying Israel's army returned fire with tanks and light weapons — but denied its troops had entered Lebanon.

The Israeli troops crossed the heavily guarded border fence, the Israeli army said, but remained south of the international border and within Israeli territory, which stretches beyond the fence line.

The Israelis said the army was clearing land, searching for explosive devices planted by Hezbollah in violation of the cease-fire, which also requires international peacekeepers to prevent new attacks on Israel.

Hezbollah has denied the allegation, saying the explosives were planted before the war.

It was the first cross-border shooting since shortly after an Aug. 14 cease-fire ended the fighting between Israeli forces and Lebanese Hezbollah militants, and after Israeli troops withdrew to their side of the border in September.

A clash involving an Israel's commando raid on the Bekaa Valley town of Boudai deep inside Lebanon occurred five days after the cease-fire and left an Israeli officer killed. Israel said that was an attempt to interdict Hezbollah weapons shipments.

McDowell, of the U.N. Interim Force in Lebanon, or UNIFIL, said Wednesday's exchange was "initiated" by the Lebanese arm and that the Israeli bulldozer crossed the "technical fence" to clear mines. His statement did not clearly define whether the Israelis had crossed the border, but indicated the Israelis were still on their side when the shooting erupted.

The shooting lasted for a few minutes, military officials said. McDowell said the clash ended before midnight.

In New York, U.N. associate spokesman Farhan Haq said the U.N. peacekeepers in south Lebanon talked to both sides in an effort to prevent any escalation.

"The latest news is that things seem to have calmed down," Haq said.

Shortly after the clash, the Israeli army said all its forces had withdrawn south of the border fence.

About 15,000 Lebanese troops deployed to south Lebanon under the U.N. resolution that included the cease-fire which ended the fighting. More than 1,000 people died in Lebanon and about 150 in Israel in the 34-day war. Also, 34 Lebanese soldiers were killed, many in Israeli airstrikes against army positions and radar installations. Israel has accused the Lebanese army of aiding the Hezbollah.

Hezbollah officials had no immediate comment on the incident.

Israel has sent warplanes repeatedly over Lebanon on reconnaissance flights, which the UNIFIL termed as a violation of the cease-fire resolution.

Also, another demand from the U.N. cease-fire resolution — the release of two Israeli soldiers whose capture by the Shite Hezbollah in a cross-border raid July 12 ignited the summer war — remains unfulfilled. Hezbollah has said it will only release them in exchange for Lebanese prisoners held by Israel. The United Nations has been quietly mediating since the cease-fire.

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