Israeli Defense Minister Ehud Barak threatened Thursday to intensify military operations in the Gaza Strip if fighters continue using the Palestinian territory for rocket attacks on southern Israel. Earlier in the day, Israeli troops supported by tanks, artillery and fighter jets raided Gaza, killing six Palestinian gunmen, according to Palestinian and news service accounts.

Also, a 42-year-old Palestinian high school chemistry teacher was killed when a shell hit a school just before classes started in the morning, said Jamil Suleiman, director of the hospital in the Gaza village of Beit Hanoun. Three 16-year-old Palestinian boys, all students, were wounded, Suleiman said.

Israel denied targeting the school, saying it was firing at rocket teams that use the border village as a base for attacks on Israel. On Thursday, fighters fired at least seven rockets at the southern Israeli town of Sderot, wounding one person, the Israeli military said.

Fighting between Israel and the armed Hamas movement that controls Gaza has increased since mid-January, when militants responded to a visit by President Bush to Israel with stepped-up launches of their handmade Qassam rockets and Israel intensified airstrikes.

Israeli authorities say they have held off on larger ground or air offensives so as to minimize casualties. That could change, Barak said Thursday. "If the Qassam fire continues, we will intensify our activity, and the other side's losses, until we resolve the Qassam rocket problem," he said during a visit to a military base in Israel's north.

Israeli troops entered Gaza early Thursday, drawing out Palestinian fighters in gun battles. Hamas said gunfire and Israeli missiles killed five of its men. A fighter from the Islamic Jihad group was also killed, news agencies said.

Fighting between Hamas and its political rival, Fatah, last summer broke up a unity government that the two had formed and left Hamas in charge of Gaza and Fatah in charge of the West Bank. The recent escalation of hostilities has overshadowed peace talks that had resumed between Israel and Fatah after Bush's visit.

Israeli government spokesman Mark Regev this week urged Palestinian Authority President Mahmoud Abbas, a Fatah leader, to bring what Regev called "rogue elements" in the West Bank under control. Hamas asserted responsibility for a suicide bombing Monday that killed a 73-year-old Israeli woman in southern Israel. Hamas said it had dispatched the two attackers from the West Bank town of Hebron.

Until Monday's bombing, Hamas had not claimed to have carried out a suicide attack in Israel since 2004.

Israel's Defense Ministry on Thursday directed the country's Infrastructures Ministry to proceed with a small cut in electricity to Gaza, the first of a possible series of power supply reductions meant to pressure Hamas to stop its rocket attacks. Since last month, Israel has sharply reduced shipments of fuel and other goods to Gaza.

"The combination of military action on the one hand and sanctions on Gaza on the other . . . will eventually bring the Qassam fire to a halt," Barak said at the military base.

On Jan. 23, fighters eased the pressure of the sanctions in Gaza by blowing up miles of the territory's border fence with Egypt, allowing hundreds of thousands of Gaza residents to cross over and shop in that country. Egypt resealed the border Sunday.

On Thursday, Egyptian Foreign Minister Ahmed Aboul Gheit warned Gazans not to breach the wall again. "Whoever breaks the border line shall have his foot broken," Egypt's state news service quoted him as saying.

Aboul Gheit also said Egypt was working diplomatically to ease restrictions on Gazans entering and leaving the strip through legal Gaza-Egypt border crossings. He urged Hamas to halt attacks on Israel in the meantime, saying rockets "lost in the sands of Israel" only give Israel an excuse for attacks on Gaza.