The militant group Hamas warned Thursday that it would strike Israelis everywhere in retaliation for the killing of 15 Palestinians during a search for militants in the bloodiest day of fighting in the Gaza Strip in 16 months.

Group leader Mahmoud Zahar said on Israel Radio that the military wing had urged all of its cells in Gaza and the West Bank to attack. Similar calls in the past have been followed within days -- sometimes hours -- by homicide bombings in Israel.

"The military wing expects that the operations will be there (Israel) at any time," Zahar said.

Some analysts linked the stepped-up violence to Prime Minister Ariel Sharon's proposals for a unilateral pullout from most of Gaza. Many Israelis, including the military's intelligence chief, are concerned such a move might be viewed by Palestinians as a sign of weakness. Analysts warned more military action in the territory could lie ahead.

If peace talks fail, Sharon has said he will remove up to 17 of 21 Israeli settlements in Gaza and impose a temporary boundary in the West Bank. Israel captured the two areas, where the Palestinians hope to declare an independent state, in the 1967 Mideast war.

Senior military officers have said privately that they believe pressure needs to be increased on the Palestinians ahead of any withdrawal.

Israel's army will "continue to flex its muscles" with operations in the Gaza Strip, while Hamas will continue its efforts to take control of the area, wrote military analyst Ze'ev Schiff in the Haaretz newspaper Thursday.

Another Hamas leader, Abdel Aziz Rantisi, posted a statement on the group's Web site on Thursday, promising Israelis the group would take revenge. In more than three years of fighting, 455 people have died in homicide bombings carried out by Hamas and other militant factions.

"You will weep blood rather than tears," Rantisi said. "You will not escape punishment for such a heinous crime."

The fiercest fighting on Wednesday took place in the Shajaiyeh neighborhood of Gaza City. Twelve militants, including the son of a prominent Palestinian leader and a senior Hamas activist, were killed and more than 40 were wounded, Palestinian doctors said.

In a separate raid in the Rafah refugee camp along the Gaza-Egypt border, troops killed three Palestinians, including a militant, as they searched for tunnels used for arms smuggling. The forces demolished three houses and razed citrus and olive groves.

The army said it had entered the Gaza City neighborhood to search for militants who fire rockets at nearby Jewish settlements. It said the fighting broke out after militants fired missiles at Israeli tanks.

Among the dead were Mohammed Hilles, 18, the son of Ahmed Hilles, the top leader of Yasser Arafat's Fatah faction in Gaza, and senior Hamas activist Hani Abu Skhaila.

The fighting was the deadliest in Gaza since 19 Palestinians were killed in clashes in Khan Younis on Oct. 7, 2002.

Later Wednesday, thousands of people marched in funeral processions for some of the militants. Masked men in military-style uniforms carried bodies on stretchers, while others fired machine guns into the air and called for revenge.

After the fighting died down, four small rockets were fired into Israel, causing no injuries or damage, the army said. Later, militants fired a mortar at a Jewish settlement in Gaza, badly damaging a house and lightly injuring a settler.

Hamas, which has carried out dozens of homicide bombings in Israel during more than three years of fighting, said the rocket attacks were the first part of what it promises will be a painful retaliation to the Israeli incursion.

The group's military wing issued a statement calling on all its cells in the West Bank, Gaza and Jerusalem to carry out "huge martyrdom operations ... everywhere in Palestine," referring to homicide bombings in the West Bank, Gaza and Israel.

Palestinian Prime Minister Ahmed Qureia said the Gaza incursion endangered efforts to revive long-stalled peace talks.

"We believe that these killings must stop immediately if the peace process is to go forward and bring results," Qureia said in Rome after talks with Italian Foreign Minister Franco Frattini.