Israeli combat helicopters have attacked a Palestinian militant group in the West Bank town of Nablus, killing at least eight people.
Israeli helicopter gun ships blasted offices of the militant Islamic group Hamas in Nablus.
Two senior members of Hamas' political wing, Jamal Mansur and Jamal Salim, died in the attack. Two children, who were brothers, were also killed.
After the attack, Palestinians poured into the streets of Nablus crying in anguish and calling for revenge. Palestinian officials are describing the killings as a "massacre," and the Palestinian Authority called for a two-day period of mourning.
Speaking in Gaza, Hamas spiritual leader Sheikh Ahmed Yassin said "the Israeli people will pay a heavy price" for the attack. Hamas has carried out numerous bombings against Israel in recent months, and has warned it is planning additional attacks.
Israeli Prime Minister Ariel Sharon released a statement saying the Hamas members had carried out acts of terror in the past, and were planning additional attacks in the future. The statement expressed regret for the deaths of the children.
Israel has carried out a series of targeted killings, described by Palestinians as assassinations, as part of a policy to halt terrorist bombings against Israelis. Palestinians say about 50 people have been targeted since the uprising against Israeli occupation began more than ten months ago.
In other violence, Palestinians say at least two people were killed in clashes with Israeli soldiers in the Gaza Strip.
Palestinians say a member of the radical Islamic Jihad movement died in a shootout with Israeli troops, although the army denied involvement in the incident. Palestinians say a security officer was also killed by Israeli fire.
More than 680 people, mostly Palestinians, have died since the conflict began last September.