"Israel has the right to defend itself from terrorist attacks," the White House said in a statement. "Hamas is a terrorist organisation that attacks civilians."

But the statement said Israel should "to consider carefully the consequences of its actions" and called on "all parties to exercise maximum restraint at this time.

"The United States is gravely concerned for regional peace and stability," the White House said.

United Nations Secretary General Kofi Annan called on Israel to "immediately end" the practice of "extrajudicial killings," calling them "violations of international law."

In a statement issued by his spokesman, Annan condemned the Rantissi assassination and urged Palestinians and Israelis to work on negotiations based on the Middle East road map for peace drawn by the UN, US, European Union and Russia.

Rantissi was killed in an Israeli helicopter strike in the Gaza Strip, less than a month after he succeeded Hamas founder and spiritual leader Sheikh Ahmed Yassin, whom Israel killed in a similar attack.

Israeli cabinet minister at large Uzi Landau expressed delight at Rantissi's assassination.

"We cannot fail to be pleased with this operation for it is necessary to continue to eliminate the terrorists as we did with Yassin," Landau told Israeli television.

A US State Department official told CNN television the United States neither gave "Israel any kind of green light" to kill Rantissi nor had advance knowledge of the attack on his car in Gaza City.

But Palestinian prime minister Ahmed Qorei charged the assassination was a "direct result" of US support for the Jewish state.

Qorei's government was outraged on Wednesday after US President George W. Bush endorsed Israeli Prime Minister Ariel Sharon's plan to pull out from Gaza but retain West Bank settlements.

"This Israeli terrorist offensive is a direct result of America's encouragement and total partiality in favour of Israel, backing its plans and offering it political cover to usurp Palestinian land," Qorei said in a statement issued in the West Bank.

Palestinian leader Yasser Arafat condemned the assassination.

"President Arafat and the Palestinian leadership repeat that the crimes of the barbaric occupiers will only reinforce our resistance," said a statement issued by the Palestinian Authority.

In an interview with Al-Jazeera television in Qatar, Hamas political chief Khaled Meshaal called on the radical Islamist movement's members in Gaza to promptly select a new leader but not to disclose his name.

Meshaal also accused "the US administration of giving its cover for the assassinations" of Rantissi and Yassin.

Europeans said the killing could hurt the peace process.

"Today's action will not facilitate a positive outcome of this process," said European Union foreign affairs chief Javier Solana.

Portuguese Foreign Minister Teresa Gouveia said the assassination "in no way contributes to the resolution of the Middle East conflict".

British Foreign Secretary Jack Straw said such assassinations were wrong and unhelpful to peace.

"The British government has made it repeatedly clear that so-called targeted assassinations of this kind are unlawful, unjustified and counterproductive," Straw said in a statement.