Israel has carried out a second helicopter attack on a car carrying Palestinians in the Gaza Strip, hours after wounding a leading militant in a similar air strike.
Rantissi was targeted by at least seven missiles
Three Palestinians from the same family were killed and at least 30 people wounded in the latest attack near the town of Jabaliya, Palestinian officials said.
US President George W Bush said he was "deeply troubled" by the helicopter attacks but said he was determined to press ahead with the peace plan for the Middle East.
The president sharply criticised an Israeli attack on a car carrying Abdel-Aziz Rantissi, considered the political leader of the militant Palestinian group Hamas, in Gaza City.
Mr Rantissi, who was lightly wounded, said Hamas would avenge the attack and continue to fight Israel until every last "Zionist" was gone.
A senior Israeli military source told the BBC that more attacks could be expected against Hamas leaders - both political and military - as long as the group continued attacking Israelis.
The violence is the most serious threat yet to the fledgling US-backed peace plan, known as the roadmap, which was endorsed by Israel and the Palestinians at a peace summit in Jordan on 4 June.
In a rare rebuke of Israel's actions, Mr Bush said the attacks on Hamas could undermine Palestinian efforts to end anti-Israeli violence.
"I also don't believe the attacks helped Israeli security," he
Escape by seconds
The attack in Jabaliya came after Palestinians fired rockets at Israel from across the border in Gaza, Israeli military sources said.
Palestinian sources said two men and a 16-year-old girl were killed when Israeli helicopters fired missiles at a car; the Israeli army said it was targeting the militants who fired the rockets at Israel, Reuters news agency reported.
In the earlier attack, Mr Rantissi leapt out of his vehicle seconds before it was hit by several missiles fired from at least two Israeli helicopter gunships.
Two Palestinians were killed and another 25 people - including Mr Rantissi's son - were injured in that attack, medical sources at Gaza City's Shifa Hospital said.
The Israeli Government admitted trying to kill Mr Rantissi, whom it said had "preached and directed murder, sabotage, terror and incitement for many years".
VIOLENCE SINCE 4 JUNE PEACE SUMMIT
10 June: Israel launches two helicopter attacks in Gaza - at least five Palestinians killed; Hamas leader Abdel-Aziz Rantissi injured
9 June: Two Palestinian militants killed trying to infiltrate Jewish settlement in Gaza
8 June: Palestinian militants attack army base in Gaza - four soldiers, three militants killed; two militants shot dead after killing soldier in Hebron; militant killed after attacking troops in Gaza
7 June: Palestinian militant killed after firing on Israeli troops in Gaza
5 June: Two Palestinian militants killed in shoot-out with Israeli police in northern West Bank
The attack came two days after four Israeli soldiers were killed in an ambush in Gaza involving Hamas militants.
The group broke off ceasefire talks with Palestinian Prime Minister Mahmoud Abbas, also known as Abu Mazen, at the end of last week after he called for an end to attacks on Israelis.
Abu Mazen said the strike on Mr Rantissi was a "terrorist attack in the full meaning of
the word because it targeted innocent people".
Observers say the attack on the Hamas leader may be designed to illustrate that Israeli Prime Minister Ariel Sharon's resolve to crack down on militants has not weakened.
Mr Sharon has come under fire from right-wing Israelis after ordering the removal of unauthorised outposts in the West Bank in compliance with the roadmap.
The move has met massive resistance from Jewish settlers, who have pledged to thwart attempts to remove them from land claimed by the Palestinians.
Associated Press reports that rabbis living in settlements denounced the move as a "crime" and a "wretched and contemptible decision".