When our civilians are brutally killed the military wing of Hamas will never keep silent and will respond in the time and place it determines

Hamas spokesman

The Palestinian Authority immediately condemned the attack as an "act of war perpetrated by the Israeli Government against the unarmed and innocent Palestinian population."

Just a few hours later, a mortar bomb was fired at the Jewish settlement of Gilo in what the Israeli army says is the first such attack from the West Bank since the latest Palestinian uprising began in September 2000.

There were no immediate reports of any casualties.

Palestinian sources said the helicopters struck a house in the town of Bethlehem belonging to the Fatah movement of Palestinian leader Yasser Arafat.

Vengeance threatened

A coalition of Palestinian groups known as the National and Islamic Forces was quoted as saying they would avenge it, and that every settler and soldier was now a target.

Hamas says it will retaliate for the attacks

Two of the men killed in the strike, named as Omar Saadeh and Taha Aruj, are believed to be activists of the Islamic militant group Hamas. The other two were Mohammed and Eshak Saadeh, relatives of Omar Saadeh.

The strike followed an overnight attack by Israeli tanks on Palestinian checkpoints in the town of Jenin and Tulkarem in retaliation for a suicide bomb attack in northern Israel.

The bomber blew himself up near a train station in the town of Binyamina on Monday evening, killing two Israelis, a male and a female soldier, and injuring at least eight others.

The tense security situation led Defence Minister Binyamin Ben-Eliezer to call off a week-long visit to the United States.

Preventive operation

An Israeli military official told Reuters news agency that the latest attack targeted a Hamas cell planning to bomb the closing ceremony of the Maccabiah games.

He said the strike was a "clear preventive operation".

Senior Hamas official Abdel Aziz al-Rantissi said the targets had gathered to await the release of a relative from an Israeli prison.

Relatives mourn those killed in the Israeli attack

"When our civilians are brutally killed the military wing of Hamas will never keep silent and will respond in the time and place it determines," Mr al-Rantissi said.

Ahmed Abdel-Rahman, a senior aide to Palestinian President Yasser Arafat, said the missile attack was "state-sponsored terrorism" that harmed Palestinian efforts to restore calm.

Targeted killings

The strike is thought to be part of a controversial policy of assassination resumed by Israel two weeks ago, as part of what it calls an anti-terror policy.

Israel has given the Palestinian Authority a list of activists it wants rounded up, accusing them of planning or carrying out attacks on Israelis.

The authority has made some arrests, but not enough to satisfy Israel, which is taking matters into its own hands, BBC Middle East correspondent Frank Gardner says.

The Palestinians have accused Israel of assassinating more than 30 militants by various methods, including bombs and helicopter gunships, since the Palestinian intifada, or uprising, began almost 10 months ago.

More than 40 people have been killed since a US-sponsored ceasefire was agreed a month ago.