It was the first time that shooting erupted across the border since shortly after an Aug. 14 cease-fire that ended a 34-day war between Israeli forces and the Lebanese Hezbollah militants.

Lebanese troops opened fire on a bulldozer after it crossed the so-called Blue Line ¡ª the U.N.-demarcated boundary ¡ª and entered about 20 yards into Lebanon, Lebanese officials said.

Israeli troops responded with tanks and light weapons, Israeli security officials said on condition of anonymity because they were not authorized to speak to the media.

The exchange occurred near the Lebanese village of Maroun el-Rass, which was the scene of heavy fighting in the summer war, in the central sector of the border.

The Lebanese military officials, speaking on condition of anonymity pending the release of a formal statement from the army command, said the Lebanese army fired volleys of machine guns toward the bulldozer.

Israeli forces responded with five anti-tank grenades that targeted an army armored vehicle and a transport jeep, the Lebanese officials said. Lebanese troops did not suffer any injuries. There was no immediate word of any Israeli casualties.

In Jerusalem, the Israeli army said shooting erupted on the Israel-Lebanon border during an Israeli operation to search for bombs planted by Hezbollah guerrillas.

The army said troops operating in Israeli territory along the frontier came under fire, and that the source of the shooting was apparently Lebanese troops nearby. When the attackers refused to quit firing, the Israeli troops opened fire at them, the army said.

The Israeli army said the Israeli force was clearing land and searching for bombs near the spot where Israeli troops discovered four explosive devices planted by Hezbollah on Monday.

The bulldozers crossed the heavily guarded border fence but remained inside Israeli territory, which extends north of the fence in that area, the army said.

The U.N. Interim Force in Lebanon, or UNIFIL , is a 12,000-strong peacekeeping force deployed to assist the Lebanese army patrol southern Lebanon to enforce the cease-fire.

"We're aware of an ongoing incident," said Liam McDowell, a UNIFIL spokesman, but added they don't have further information.

About 15,000 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.