Both U.S. and Israeli officials said, meanwhile, that Israel had not notified Washington in advance that it planned to kill Yassin with a missile after he left a mosque in Gaza.

And even the hardened U.S. 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. however, entitled to carry out extra-judicial killings."

Rep. Eliot Engel, D-N.Y., defending Israel, said, "Just as the United States 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 defense policy studies at the Cato Institute, a private research group, said Hamas' vow of vengeance against the United States means the United States may have taken on a new terrorist enemy.

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

In the past, Hamas has insisted that its fight is against Israel and has refrained from targeting U.S. citizens or interests.

Investigations regarding Hamas in this country have focused mostly on front companies and charities that funnel millions of dollars to the group.

Earlier this year, FBI Director Robert Mueller told Congress that Hamas was a threat within the United States but had not demonstrated it would act violently.