BEIRUT, Lebanon (AP) – Lebanese troops exchanged fire with Israeli forces across the border late Wednesday, for the first time since the summer war between Hezbollah and Israel, Lebanese and Israeli officials said.

The nighttime incident – sparked by an Israeli bulldozer crossing a fence – showed how tense the boundary remains nearly six months after a U.N.-brokered cease-fire ended 34 days of fighting. But the incident was not expected to undermine the overall truce.

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

The vehicle crossed the so-called Blue Line – the U.N.-demarcated boundary – and entered 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 toward the bulldozer, drawing Israeli forces to return with five anti-tank grenades that targeted an army armored vehicle and a transport jeep.

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 route of the fence.

The Israelis said the army was clearing land, searching for bombs near where explosive devices were planted by Hezbollah on Monday, under the cover of bad weather – meaning Hezbollah had violated the cease-fire. The truce accord also requires international peacekeepers to prevent new attacks on Israel.

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

U.N. peacekeepers in south Lebanon described the shootout as a “serious incident.” Peacekeepers contacted both sides, “urging them to cease hostilities with immediate effect,” said Liam McDowell, a spokesman for the force.

It was the first time cross-border shooting erupted since shortly after a ceasefire on Aug. 14 ended the fighting between Israeli forces and the 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.

Mr. McDowell, of the U.N. Interim Force in Lebanon, or UNIFIL, said Wednesday’s exchange was “initiated by the Lebanese army” 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 trying to prevent any escalation.

“The latest news is that things seem to have calmed down,” Mr. Haq said.

Shortly after the incident, 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 have 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 Hezbollah in a cross-border raid July 12 had sparked the summer war – remains unfulfilled. Hezbollah has said it will not release them except in exchange for Lebanese prisoners held by Israel. The United Nations has been quietly mediating since the cease-fire.

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Associated Press writers Matti Friedman in Jerusalem and Edith M. Lederer at the United Nations in New York contributed to this report.