WASHINGTON - The US criticism of Israel for killing a prominent Palestinian extremist reflects growing concern for the already troubled US peace effort in the Middle East and apparently also a perception of limitless US support for Israel.

After initial hesitation, President George W. Bush's administration openly criticized Israel on Monday for slaying Sheik Ahmed Yassin, a founder of the militant Palestinian group Hamas, despite attempts by Israel to draw a parallel to the US war against the al-Qaida terror network.

"We are deeply troubled by this morning's incident in Gaza," White House spokesman Scott McClellan said.

From the outset, administration officials said they had no advance warning of the missile attack in Gaza, but Hamas threatened the US as well as Israel with retaliation.

The threat is further evidence that Hamas is a terrorist organization that should be put out of business, a senior US official said.

And yet, a US counter-terrorism expert said, also on condition of anonymity, any threat against the US is taken seriously.

Earlier in the year, FBI director Robert Mueller told Congress that Hamas was a threat within the US but had not demonstrated that it would act violently.

In the initial US reaction, Condoleezza Rice, Bush's national security assistant, emphasized the US believes Hamas is a terrorist organization. And she stressed on NBC's "Today" show that "Sheik Yassin has himself, personally we believe, been involved in terrorist planning."

A few hours later, the US tone shifted. State Department spokesman Richard Boucher said the attack was "troubling," would increase tensions in the region and could make it harder to pursue peace in the Middle East.

Boucher said Israel knew of US concerns about some of its actions, including targeted killings.

In response, Israeli Foreign Minister Silvan Shalom, amid a long-planned series of meetings with Vice President Dick Cheney, Secretary of State Colin Powell and Rice, drew a parallel between Israel's struggle against extremists and the Bush administration's war on terrorism.

He said Hamas, which Yassin helped found in 1987, and al-Qaida, which the Bush administration accuses of the September 11, 2001, attacks, share the same ideology.

Shalom said he had offered Cheney intelligence data that Israel had allegedly proving Yassin's direct involvement in suicide bombings and other terrorist attacks on Israel.

Even the hardened US position did not approach the European Union foreign ministers' condemnation of Israel's attack.

Noting that Hamas had killed hundreds of Israelis, the ministers said in a statement from Brussels: "Israel is not ... entitled to carry out extra-judicial killings."

Democratic Congressman Eliot Engel, defending Israel, said, "Just as the US is justified in seeking out and targeting al-Qaida leaders such as Osama bin Laden and Ayman al-Zawahiri, the Israeli government has a responsibility to do what it can to protect its citizens against these deadly terrorists."

But Chuck Pena, director of defence policy studies at the Cato Institute, a private research group, said Hamas' vow of vengeance against the US means the US may have taken on a new terrorist enemy.

"The US cannot afford to make other countries' terrorist threats our terrorist threats," Pena said.

Stephen Cohen, president of the Institute for Middle East peace and Development, said Israel and the Palestinians were following a failed strategy that poses a growing threat to the rest of the world.

Cohen, in an interview, said Bush should not invite Israeli Prime Minister Ariel Sharon to the White House until Sharon "comes up with a proposal on what he wants to do with Gaza that is not simply a cover for more assassinations."

Debra DeLee, president of Americans for Peace Now, which describes itself as a Jewish Zionist organization, said "we fear that the assassination of Sheik Yassin will escalate the current conflict to new levels of violence and lead to the strengthening of Palestinian hardliners."

(Agencies via Xinhua)