Updated

Israel's attack on a Hamas (search) leader could make it harder for the new Palestinian leadership to combat terrorism, President Bush said Tuesday.

Bush said he was "troubled" by the Israeli helicopter attack on Abdel Aziz Rantisi in Gaza. Such incidents don't promote Israel's security, he said, and may "make it more difficult for the Palestinian leadership to fight off terrorist attacks."

"I regret the loss of innocent life," Bush told reporters in the Oval Office.

"I am determined to keep the process on the road to peace," Bush said. "And I believe with responsible leadership by all parties, we can bring peace to the region -- and I emphasize all parties must behave responsibly to achieve that objective."

Secretary of State Colin Powell, speaking in Buenos Aires, Argentina, said the attack sets back progress in peacemaking with the Palestinians.

Israel defended the targeting of Rantisi as self-defense designed to prevent more Hamas attacks on Israel, but Powell said the helicopter assault "does not contribute to the security of Israel."

Rantisi, a high-profile political leader of the Islamic militant group that claimed it participated in the killing of four Israeli soldiers on Sunday, was wounded and hospitalized. His bodyguard and a bystander were killed.

"We are deeply troubled by what happened in Gaza earlier today and we are concerned that this kind of activity will delay the kind of progress we are hoping for as we move down the road map," Powell said during a visit to Argentina.

"My colleagues back in Washington are in touch with the Israeli government and have expressed our concern to the Israeli government," he said.

Mahmoud Abbas (search), the Palestinian prime minister who pledged at a summit meeting last week with Bush and Israeli Prime Minister Ariel Sharon (search) to try to stop attacks on Israelis, accused Israel of a terrorist attack.

Hamas, meanwhile, threatened revenge "that will be like an earthquake" after the Israeli helicopter assault Tuesday.

A few hours after the attack on Rantisi, Palestinians fired six homemade rockets from northern Gaza, four of which landed in Israel. Israeli tanks and helicopters fired toward a Palestinian residential area in the northern Gaza Strip, killing three Palestinians and wounding 30, doctors said.

Israel's ambassador to the United States, Daniel Ayalon, defended the assassination attempt. He called Rantisi a "master terrorist" and said Israel would present the Bush administration with detailed information about his planned activities.

"He is one of the most extreme members of Hamas," Ayalon told the Israel Policy Forum, an American-Jewish group that advocates conciliation in the Middle East.

"He is attacking Israelis, he is attacking the peace process" as well as Abbas, Ayalon said. "Terrorism will undermine Abbas," he said of the prime minister backed by the Bush administration in preference to Yasser Arafat, who is recognized by most of the world as the Palestinians' leader.

Since the summit with Bush and Abbas in Aqaba, Jordan, Israel has dismantled 12 settlement outposts on the West Bank; released many Palestinian prisoners, including some "with blood on their hands"; turned funds over to Abbas; and eased entry of Palestinian workers to Israel, Ayalon said.

"But nothing has been done" to end Palestinian terror, the ambassador said. Referring to Abbas and his associates, Ayalon said, "They claim incapability, but they could start."

A group of American Jewish leaders met with Abbas in Ramallah on Tuesday. Jonathan Jacoby, a founding director of the Israel Policy Forum, said: "We came away feeling that Abbas is fully committed to every word he said in Aqaba in terms of fighting violence, in terms of making the road map work. I came away with a sense that Abbas represents an enormous opportunity for Israel."

Hamas participated Sunday in a rare joint operation with Islamic Jihad and the Al Aqsa Martyrs' Brigades, which Ayalon said is controlled by Arafat, to kill four Israeli soldiers at an army outpost in Gaza. On Monday, Bush offered measured criticism of that attack.

"I recognize there're going to be extremists, particularly in the Palestinian territories, that want to blow up peace. I think people are sick of it," Bush said.

The burst of violence this week prompted a flurry of calls Tuesday to Palestinian and Israeli officials from national security adviser Condoleezza Rice; U.S. Assistant Secretary of State William Burns; Elliott Abrams, who heads the Middle East desk at the National Security Council; acting U.S. Consul-General Jeffrey Feltman; and U.S. ambassador to Israel Daniel Kertzer.

Bush did not make calls himself, White House spokesman Ari Fleischer said.

"There's a moment when the president will decide when it's appropriate for this to be a presidential contact," Fleischer said.