NABLUS, West Bank ? Israeli troops entered the narrow alleyways of the Old City of Nablus early today, residents said, after about 150 armored vehicles moved into the West Bank city after midnight.

Tanks and armored personnel carriers took up positions in the middle of the city, residents said, and surrounded the Old City. Soldiers left their vehicles and moved into the alleys, trading gunfire with Palestinians. Two Palestinians were killed, witnesses said.

Israel’s military said in a statement it was carrying out the incursion because Nablus was the focus of activity of “terror groups,” including Hamas, which had claimed responsibility for Wednesday’s bombing at Hebrew University. Seven people, including five Americans, were killed in the attack.

Nablus residents said the tanks entered the city from four directions, the largest incursion since Israel took over seven of the eight main West Bank cities and towns starting June 20. The invading force included bulldozers, witnesses said, indicating that the Israelis planned to tear down buildings.

Nablus had been under tight curfew, its residents ordered to stay in their homes for six weeks with only a few short breaks, but the people successfully defied the confinement orders for three days this week before Israeli troops moved back into the middle of the city on Wednesday.

Earlier, Israel Radio reported that the Israeli military commander had signed orders to expel two relatives of Palestinian attackers from the West Bank to the Gaza Strip. The radio report said the two had 12 hours to appeal.

Israel’s Security Cabinet this week considered new measures to stop or deter suicide bombers, including expelling relatives of bombers to Gaza and tearing down family homes. The Israelis felt that if potential bombers knew their families would suffer as the result of their actions, they would reconsider.

In the Gaza Strip, one Palestinian was killed and two wounded in exchanges of fire when Israeli troops moved to destroy structures along the Gaza border with Egypt.

Israeli troops control seven of the eight main Palestinian centers in the West Bank, moving in after back-to-back suicide attacks in Jerusalem June 18-19. The university bombing and a suicide attack the day before were the first bombings in Jerusalem since the incursions.