White House national security advisor Condoleezza Rice said in a round of interviews on U.S. television stations that the United States did not have advance warning of the assassination.

"It is very important that everyone step back now and try now to be calm in the region. There is always a possibility of a better day in the Middle East and some of the things being talked about by the Israelis ... might provide new opportunities," Rice told NBC's "Today" show.

"I would hope that nothing will be done to preclude those new opportunities from emerging," she added.

Israeli security sources said Israeli Prime Minister Ariel Sharon personally ordered and monitored the helicopter attack against the paralyzed cleric outside a Gaza mosque.

Pressed on whether the United States had played a role in Yassin's death or whether Sharon had called President Bush to tell him that Israeli forces planned the assassination, Rice replied: "He did not."

Hamas said it believed Washington had given the green light for Yassin's assassination but Rice disagreed. "No, of course not," Rice told CBS.

She added: "Let's remember that Hamas is a terrorist organization and that Sheikh Yassin himself has been heavily involved in terrorism."