WASHINGTON — After initially hesitating, the Bush administration criticized Israel on Monday for killing Sheik Ahmed Yassin, a founder of the militant Palestinian group Hamas. "We are deeply troubled by this morning's incident in Gaza," the White House spokesman said.

The criticism appeared to reflect both concern for the already troubled U.S. peace effort in the Middle East and a judgment that the Bush administration should reassure European and Arab governments that its support for Israel is not limitless.

Administration officials said they had no advance warning of the killing, which has raised fears of possible retaliation against the United States. Hamas directly threatened the United States — something it usually does not do — saying America's backing of Israel made the assassination possible.

In the initial U.S. reaction, Condoleezza Rice, President Bush's national security adviser, emphasized that the United States 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 U.S. tone shifted. State Department spokesman Richard Boucher said the attack was "deeply troubling," would increase tensions in the region and could make it harder to pursue peace in the Middle East.

"I think if you ask the Israelis, you'll find that they take these actions with full knowledge of the United States' positions, including our concern about some of these actions, our position on actions that have been taken in the past, but also knowing that we do not approve of actions like this," Boucher said.

The White House, which had urged restraint but had not directly criticized Israel, also hardened its reaction. "We are deeply troubled by this morning's incident in Gaza," spokesman Scott McClellan told The Associated Press after steering clear of criticism in his regular daily briefing earlier.

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 Sept. 11, 2001, attacks, share the same ideology.

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