Israeli forces killed eight Palestinians, including two policemen early Tuesday, during their latest incursion into Palestinian territory, and Israeli Prime Minister Ariel Sharon vowed to wage a "massive campaign" against Islamic extremists in Gaza.

The two Palestinian policemen were killed when Israeli troops stormed the Palestinian government headquarters in the West Bank city of Hebron, Palestinian security officials said. Eight other policemen were wounded .

The Israeli military said soldiers entered Hebron to operate against the "terrorist infrastructure" there. A military statement said a "large number" of suspects had been arrested. Also, soldiers discovered an explosives laboratory. Residents said soldiers declared a curfew over the city.

Six other Palestinians were killed by helicopter missile strikes Monday, including four members of the militant group Hamas, the group said. Israel said a senior Hamas activist was among those killed.

Israeli troops early Monday encircled Palestinian leader Yasser Arafat's Ramallah headquarters for the third time this month. The latest turmoil came as President Bush called for a new Palestinian leadership — one "not compromised by terror" — and set stiff conditions for a Palestinians state.

Sharon echoed Bush's call for new leadership, but Arafat aide Saeb Erekat said the demand was "not acceptable." Arafat himself welcomed Bush's ideas as a "serious effort to push the peace process forward" in a statement that ignored the call for a leadership change.

As progress is made on security matters, Bush said Israel should remove its forces from West Bank areas under Palestinian control before violence erupted in September 2000. Israeli forces now control six of the eight main Palestinian cities and towns in the West Bank: Jenin, Nablus, Qalqiliya, Ramallah, Tulkarem, and Bethlehem. Hebron could be the seventh, leaving just Jericho.

At least 600,000 Palestinians in the West Bank are confined to their homes by army curfews, although restrictions were briefly lifted in Nablus and Qalqiliya so residents could shop and go to school.

A major six-week military offensive earlier this year that was aimed at wiping out militias behind terror attacks also focused on the West Bank, leaving the Gaza Strip — where top militia leaders are based — largely unscathed. This time, however, Sharon warned Gaza would not be spared.

"We are preparing massive activity against Hamas in the Strip," Sharon said, adding that it had begun with the Israeli helicopter missile strikes on two cars Monday morning in the southern Gaza Strip. He did not say what might follow.

Six Palestinians were killed in the missile strikes in Rafah and five Palestinians were injured. Four of the dead belonged to Hamas, the militant group said.

Israel said it targeted and killed Yasser Rizik, a "senior Hamas activist" who was allegedly behind a January attack that killed four Israeli soldiers.

After the missile strike, Hamas renewed its threats against Israel as Israeli and Palestinian authorities cracked down on the group, which has carried out scores of suicide bombings against Israelis.

"We emphasize our right to continue the jihad [holy war] and resistance, and to intensify the martyrdom operations," Hamas said in a statement. "Martyrdom operations" is the term Hamas uses for suicide attacks.

Palestinian police also surrounded the Gaza City home of Hamas spiritual leader Sheik Ahmed Yassin early Monday, and Palestinian authorities said he was under house arrest. However, Yassin invited journalists inside later in the day and said he was unaware of any such order.

"No one has informed me about any house arrest decision, and there is no agreement — and there is no preparation — for any house arrest," Yassin said.

After brief scuffles between Hamas supporters and police, Palestinian police pulled back from near Yassin's home. Masked Hamas gunmen patrolled the sandy streets outside the house and about 60 people gathered nearby with a Hamas flag.

Israeli forces arrested 10 Palestinians in the Hebron area Monday, most of them Hamas members. Palestinian authorities also have said they'd arrested dozens of Hamas members in Gaza in recent days.

In Ramallah, the Palestinian political headquarters in the West Bank, Israeli soldiers took up positions around Arafat's offices, where a bulldozer quickly barricaded the front gate and nearby streets with stones and debris.

More than 20 tanks were parked on the streets that surround Arafat's compound as of Monday afternoon, and forces fanned out throughout Ramallah, imposing a curfew on residents of the city and adjoining al-Bireh.

Arafat was inside the compound with security aides and was not harmed.

"The Israeli government, with these continuous attacks, has revealed to the whole world its real intentions," Arafat said in a statement issued by the Palestinian news agency Wafa. "Nothing can weaken our belief in our legal right to have our own independence and freedom, and all this Israeli aggression will not achieve any political results."

The army said it had arrested 13 Palestinian policemen for questioning, taken up "strategic points" in Ramallah, and had placed the area under curfew. Palestinians detonated explosives, slightly injuring one soldier, the army said. The streets were deserted except for Israeli jeep patrols.

Israel forces have besieged Arafat's compound on and off since last December. His offices now consist of battered and scorched buildings that cover a full city block.