Palestinians vowed to avenge the missile strikes
Israel has defended its two deadly Gaza missile attacks amid charges that they were timed to scupper the fragile peace process.
Prime Minister Ariel Sharon's spokesman told the BBC that Israel would not hesitate to act "if the Palestinian Authority fails to take action against ticking bombs".
Palestinian leaders and left-wing Israelis say the air strikes - one of which targeted militant Hamas leader Abdel-Aziz al-Rantissi - were intended to undermine US-led efforts to revive the peace process.
Egyptian intelligence chief Omar Suleiman has been trying to broker a ceasefire deal with Hamas.
Last week the militant group broke off talks with Palestinian Prime Minister Mahmoud Abbas, accusing him of making too many concessions to Israel.
Mr Suleiman met Mahmoud Abbas - also known as Abu Mazen - and Palestinian leader Yasser Arafat in the West Bank on Wednesday.
Abu Mazen has been struggling to persuade Palestinian militant leaders to declare a truce.
In the first helicopter strike, in Gaza City on Tuesday, Mr Rantissi was lightly wounded, but two people died.
Hours later three people died in a second missile strike near Jabaliya.
The BBC's David Chazan says Israel's failed attempt to kill Mr Rantissi has shot a huge hole in the peace process just as it was getting under way.
According to Michael Tarazi, a legal adviser to the Palestine Liberation Organisation, the attack was timed "to undermine Prime Minister Abu Mazen in his efforts to bring Hamas and [Islamic] Jihad on board".
Israeli leftists say Israel could have tried to assassinate Mr Rantissi long before its recent resumption of dialogue with the Palestinian Authority.
Mr Rantissi often appears on television and does not try to keep his whereabouts secret.
Mr Sharon has ordered his aides to turn over intelligence to US officials which would back up accusations Mr Rantissi had been planning attacks.
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
In another development connected with the peace process, Israel's High Court ordered the army not to evacuate a number of settler outposts until it considered a legal challenge on Wednesday.
The army had begun dismantling some - mainly uninhabited - outposts as demanded by the US-backed peace plan known as the roadmap.
Settlers have resisted on the ground as well as in the courts, but there have been no reports of serious violence.
US President George W Bush - who wants both sides to implement the peace plan known as the roadmap - said he was "troubled" by the missile strike on Mr Rantissi's car.
"I also don't believe the attacks helped Israeli security," he said.
However, he added: "I am determined to keep the process on the road to peace."
Hamas has vowed an "earthquake" of revenge for the attack on Mr Rantissi, while Abu Mazen condemned it as a "crime".
The violence is seen as the most serious threat yet to the fledgling US-backed peace plan, which was endorsed by Israel and the Palestinians at a summit in Jordan on 4 June.
Mr Sharon said Israel would continue to "fight the heads of the extremist terrorist organisations - those who initiated, those who fund and those who send terrorists to kill Jews".
Speaking from his hospital bed in Gaza, Mr Rantissi vowed to continue fighting Israel and called on all Palestinian militant groups to "kill Israeli political leaders".
In the West Bank, Jewish settlers stepped up their attempts to stop the army removing any more unauthorised outposts, after troops dismantled about 10 sites - as demanded by the roadmap.
At an outpost near the settlement of Beit El, settlers furnished hitherto empty caravans and vowed to turn the hilltop into a proper neighbourhood.
Israel's Supreme Court granted a temporary injunction blocking the demolition of the Havat Gilad outpost, after settlers launched a legal challenge against its removal.