U.N. Secretary-General Kofi Annan and many world leaders condemned Monday's killing of Yassin, the most prominent Palestinian targeted by Israel in 3 1/2 years of fighting. The Bush administration said it was "deeply troubled" by the attack and that it had no advance warning.

"We will get revenge for every drop of blood that spilled," said Salman Bdeiri, a Hamas supporter crying near the mosque where Yassin prayed shortly before being killed by an Israeli airstrike.

Israel sealed off the West Bank and Gaza, banning Palestinians from Israel, and placed its security forces on high alert.

Hamas founder Sheik Ahmed Yassin

Later Monday, Palestinian militants fired several homemade rockets and mortar shells at Israeli targets in and near Gaza. To the north, Hezbollah guerrillas fired an anti-tank missile at Israeli troops along Israel's border with Lebanon.

Israel quickly responded to both attacks. It struck at Hezbollah with planes and artillery fire. No casualties were reported. Israel sent tanks into northern Gaza near the town of Beit Hanoun. Israeli security officials said the purpose of the operation was to prevent further rocket fire.

The Yassin assassination was part of Prime Minister Ariel Sharon's effort to crush Hamas ahead of a possible Israeli withdrawal from Gaza. However, the killing was seen as a major gamble that could galvanize the Palestinians behind Hamas. Rival Palestinian militant groups immediately pledged solidarity with Hamas.

The missile strike dealt what could be the final blow to the stalled U.S.-led "road map" peace plan. It also angered Egypt and Jordan, moderate Arab states whose tacit support Sharon needs for any unilateral withdrawal from Gaza.

Since Yassin founded Hamas in 1987, the group has killed hundreds of Israelis in scores of attacks. Hamas wants to destroy the Jewish state and replace it with an Islamic one.

For the first time Monday, Hamas threatened the United States and suggested it might seek outside help in carrying out revenge attacks.

"The Zionists didn't carry out their operation without getting the consent of the terrorist American administration and it (the United States) must take responsibility for this crime," Hamas said in a statement. "All the Muslims of the world will be honored to join in on the retaliation for this crime."

In the past, Hamas has refrained from targeting U.S. citizens or interests, instead focusing on fund-raising and recruitment within the United States, according to senior U.S. law enforcement officials.

Homeland Security Secretary Tom Ridge was heeding the Hamas terror warning.