Israel launched an airstrike on Gaza early Wednesday after its troops clashed with Hamas militants who fired mortars into Israel, leaving six Palestinians dead. It was the first battle since a June truce mostly quieted violence in the volatile territory.

The Israeli army said the clashes erupted late Tuesday after its forces uncovered a tunnel in central Gaza that militants planned to use to abduct Israeli soldiers. It said a special army unit headed to the area to destroy the tunnel. One Palestinian was killed in fierce gunbattles that ensued.

Hamas then fired mortars across the Gaza border into southern Israel and Israel answered with the airstrike in the early hours of Wednesday, killing five suspected Palestinian militants, Israeli and Palestinian officials said. The army said the airstrike aimed at the mortar launchers and hit them.

The clashes threatened to unravel the cease-fire between Israel and Palestinian militant groups reached in June after months of indirect negotiations. The deal halted a deadly cycle of Palestinian rocket attacks and Israeli reprisals, though sporadic rocket attacks on southern Israel have persisted.

There has been only one other fatality since the truce, in July when Israeli troops shot and killed a teenage Palestinian militant along the border with Gaza.

The Islamic militant Hamas, which controls Gaza, quickly vowed revenge.

"Our response will be harsh, and the enemy will play a heavy price," Hamas said in a statement on its military wing's Web site.

Taher Nunu, a Hamas government spokesman, said the group considered the Israeli airstrike a violation of the truce.

"This is a serious breach of the truce understandings reached through Egyptian mediation," he said in an e-mail message to reporters. "We consider this the most serious in a string of breaches."

The Israeli army claimed the move against the tunnel did not violate the truce, but instead was a legitimate step to remove an immediate threat.

A top military official said troops had discovered a "ticking tunnel," which was about to be used to abduct an Israeli soldier. The official, who spoke on condition of anonymity because of the sensitive nature of the matter, said the tunnel was dug from inside a Gaza home and showed that Hamas was using civilians for cover.

The official said Israel did not intend to break the truce, noting that some 60 mortar have been fired since the cease-fire and Israel has chosen not to respond.

Hamas also insisted it had not violated the truce and was acting to prevent an Israeli incursion.

Moaiya Hassanain, a Palestinian Health Ministry official, said the initial gunbattles killed one Palestinian and wounded three, including one woman. Residents identified the man killed as a Hamas militant. Hassanain said rescue officials were having a hard time reaching the site of the fighting and getting precise information on casualties.

Israel has maintained a blockade of Gaza, where 1.4 million Palestinians live, since Hamas overran the territory in June 2007, seizing it by force from the rival Fatah faction.