Bloody revenge was vowed against Israel by the Islamic militant group Hamas Saturday after its West Bank military operations leader and two other activists fell victim to a missile attack.

"Sharon, wait, revenge is coming soon," a reference to Israeli Prime Minister Ariel Sharon, was chanted by tens of thousands of Palestinians who joined the funeral procession, firing guns in the air, and waving green Hamas flags.

Teissir Imran, the Hamas leader in Nablus, proclaimed to the crowd that "Sharon opened the door to hell, for himself and his people."

Hours later, an mortar attack in the Gaza Strip killed one Israeli and wounded another. The attack took place at Kfar Darom, a Jewish settlement, according to a military official.

Even though the United States continues to work on peace relations, it is anticipated that the already shaky region will plummet into further turmoil since the killing of Mahmoud Abu Hanoud, a suspected mastermind of several major suicide attacks in Israel since 1997.

Abu Hanoud, 34, was a "professional terrorist" responsible for the deaths of scores of Israelis, Israeli Foreign Minister Shimon Peres said.

Abu Hanoud had planned to carry out more attacks, and Israel acted in self defense in killing him, Peres said on Israel Radio.

Nabil Abu Irdeineh, an adviser to Palestinian leader Yasser Arafat, accused Israel of trying to sabotage U.S. peace efforts by killing Abu Hanoud.

Two mediators, Assistant Secretary of State William Burns and retired Marine Corps Gen. Anthony Zinni, are expected to arrive Monday in an attempt to revive a truce deal and peace talks.

The targeted killings came at a time of growing tension. On Thursday, five boys from the Khan Younis refugee camp in the Gaza Strip were killed when they stepped on some explosives.

Israel admitted Saturday its forces had planted the bomb in a place from which Palestinian militants had been firing mortars, anti-tank weapons and automatic weapons at nearby Jewish settlements and army positions.

"From an initial investigation ... the possibility emerges that the children were killed because they were playing with a bomb which Israeli soldiers had planted in the sandbagged position which had been used for terrorist attacks on our forces," the army statement said, adding that the position was outside the populated area of the camp.

Abu Hanoud, his deputy Ayman Hashaykah and Hashaykah's brother, a lower-ranking Hamas activist, were driving in a van between the West Bank towns of Nablus and Jenin on Friday evening when their vehicle was hit by several Israeli missiles fired from a helicopter, Palestinian security officials.

Mustafa Abu Hanoud, a brother of the Hamas leader, said he was told by a witness that the van was hit by six missiles and heavy machine gun fire. Abu Hanoud's body was torn apart in the attack, the brother said.

The Hamas military wing, Izzedine al Qassam, said in a leaflet that it is "committed to avenging the blood of one of our leaders."

Hamas declared a three-day period of mourning and asked Palestinians in the West Bank and Gaza Strip to observe a strike Saturday.

Hamas has carried out a series of suicide bombings in Israel in recent years, including several that came as revenge for the killings of senior members by Israel. In response to Israel's killing of a top Hamas bombmaker in 1996, Hamas carried out four major suicide bombings that killed scores of Israelis.

Israel has tried to capture Abu Hanoud in the past.

In August 2000, three Israeli undercover troops were killed by friendly fire in a botched attempt to arrest him in his home village of Assira al-Shamaliya near Nablus. Abu Hanoud fled and was taken into custody by the Palestinian Authority.

Last May, he was injured when Israeli warplanes retaliating for a suicide attack at a shopping mall in Israel bombed the Nablus prison where he was being held. Israel said he was released after the air strike.

In the Gaza Strip, meanwhile, Palestinian school children observed a moment of silence Saturday for the five boys killed earlier in the week. The five, ages 7 to 14, were heading to school when the blast went off.

Palestinian security officials have said the booby-trapped device was planted by Israeli forces targeting Palestinian gunmen who have been shooting from the area at nearby Israeli settlements.

Several Israeli politicians called for a swift investigation by the military. "This must be investigated thoroughly, and conclusions must be drawn, both to know what happened, and so that such things don't happen in the future," Peres said. "We cannot let this pass."

The Associated Press contributed to this report.