At least nine Palestinians have been killed as Israeli tanks and troops raided the West Bank city of Hebron. The Israeli army says at least 17 Palestinians were arrested.
Under cover of darkness, Israeli troops backed by helicopters entered Palestinian-controlled areas of Hebron.
Palestinian witnesses say a missile fired from an Israeli combat helicopter hit a house, killing a gunman and four other people. They also say two other Palestinians who rushed to the scene were killed by helicopter fire.
The Israeli army says it carried out house-to-house searches for militants and confiscated explosive materials, automatic weapons and missiles.
The military says among those arrested are members of the militant groups Hamas, Islamic Jihad and the military wing of Palestinian leader Yasser Arafat's Fatah movement.
In downtown Hebron, Israeli troops lined up dozens of Palestinian men against a wall, handcuffing and blindfolding them.
Soldiers in Hebron have declared a curfew, confining Palestinians to their homes.
Israeli Defense Minister Benyamin Ben-Eliezer says the purpose of the incursion is to damage the "terrorist infrastructure" in Hebron. "We have no intention not to capture, not to recapture, not to remain there. Our intention is to get there, to do what we have to do and to get back," he said.
The operation was allegedly a response to an attack by Palestinian militants last Saturday on the nearby Jewish settlement of Adora. Palestinian gunmen infiltrated the settlement, killing four Israelis, including a five-year-old girl.
The Islamic militant group Hamas claimed responsibility for that attack.
Israel says it has information that militants from Hebron are planning more attacks.
Palestinian negotiator Saeb Erekat says Israel acted in bad faith by sending troops into Hebron, calling it a slap in the face to the Palestinians.
Israel launched massive incursions into most Palestinian population centers in the West Bank late last month in response to a series of suicide bombings that killed dozens of Israelis.
The military has pulled out of most areas, but continues to surround many Palestinian cities with Israeli tanks and soldiers.