This attack comes under American sponsorship. It is a war between us and them

Abdel-Aziz al-Rantissi Hamas political leader

According to some estimates, 50,000 people gathered in the West Bank city of Jenin for the funeral of Mahmoud Abu Hanoud, who died along with two other members of the militant Hamas group when an Israeli helicopter gunship fired a missile at their car near the West Bank town of Nablus on Friday.

In Gaza City, more than 10,000 demonstrators took to the streets for a rally addressed by Hamas political leader Abdel-Aziz al-Rantissi.

Hanoud's assassination came just after the deaths of five young Palestinian children in an explosion, and Israel on Saturday admitted the possibility that it had been responsible.

Correspondents say many of Saturday's mourners pumped bullets into the air as they pledged that revenge against Israel would "be very strong and very soon".

"This attack comes under American sponsorship. It is a war between us and them," Mr al-Rantissi told the crowd in Gaza.

"Our response will come without a doubt, and God willing it will be painful."

Pizzeria attack

The Israeli Government said Mr Hanoud "was implicated in a long series of bombing attacks against Israeli citizens".

Mahmoud Abu Hanoud: Latest victim of Israel's "assassination policy"

He had allegedly planned the suicide bombing of a Tel Aviv disco in June which killed 20 people, mostly teenagers, and also the attack on a Jerusalem pizzeria in August, which left 15 people dead.

Mr Hanoud had been planning further suicide attacks on Israeli territory, the government said.

But Israel has been widely criticised for targeting and killing people it suspects of being involved in attacks on Israeli civilians.

A BBC correspondent in Jerusalem says this latest incident will be hailed as a major success by the Israeli military, but is sure to undermine the efforts of US mediators who are due to arrive in the region in the next few days.

Israeli booby-trap bomb

The killing of the three men and four other Palestinians on Friday followed the deaths of five Palestinian schoolboys in an explosion in the Gaza Strip.

Akram Naim al-Astal was among the dead schoolboys

The Palestinian Authority accused Israel of planting a booby-trap bomb which the children triggered on Thursday, and the Israeli army on Saturday appeared to confirm that it was one of its devices.

"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," an army spokesman said.

Tension has been high in Gaza for months, with Israeli tanks in position to protect the Jewish settlers who have made their homes on Israeli-occupied land nearby.

The recent Palestinian deaths follow an announcement by Israeli Prime Minister Ariel Sharon that he will visit Washington next month for talks with US President George W Bush.

An opinion poll published in the Israeli newspaper Maariv suggests that 55% of Israelis believe their government should try harder to reach a peace settlement, with 59% backing an independent Palestinian state.