Five European-based organizations believed to raise money for the terrorist Palestinian group Hamas (search) will no longer have access to their U.S. assets under an order to the Treasury Department (searchissued Friday.

The move, being carried out by the Treasury Department's Office of Foreign Assets Control (search), also targets six top Hamas leaders.

U.S. banks will be told to seize all funds in accounts that are held by the individuals' and groups' names. It is unclear whether any funds are in American banks.

President Bush said he was cutting off those groups in response to Hamas' Tuesday homicide attack on a packed bus in Jerusalem (searchthat killed 20 people, including six children and five Americans. 

"Hamas has reaffirmed that it is a terrorist organization committed to violence against Israelis and to undermining progress toward peace between Israel and the Palestinian people," Bush said in a statement.

Bush also called upon other nations who want to see peace in the Middle East to recognize Hamas as a terrorist organization and take actions to deny it support.

"I will continue to work with leaders in the neighborhood to encourage them to cut off the money and the aid and the help that goes to these terrorist organizations, all of which aim to destroy any hope for peace," Bush said while taking questions from reporters traveling with him in Washington state, where he was discussing environmental and energy issues and attending a fund-raiser for his re-election campaign.

A senior administration official, speaking on condition of anonymity, said it was the first effort to block Hamas' assets or funding sources outside the United States. However, similar action has been taken worldwide against Al Qaeda (search) and other terrorist networks.

In May, the Bush administration cut off assets belonging to the Al Aqsa Foundation (search), a group with offices in Germany, Belgium, Sweden and Pakistan that are believed to fund Hamas activities.

The official said several European countries appear supportive of the asset freeze, but that the administration was waiting to see the official response.

The bus bombing prompted the Israeli military to target a Hamas leader Abu Shanab (search), whose car was struck in a retaliatory missile fire on Thursday. Shanab and two of his bodyguards were killed.

Hamas responded by declaring a seven-week-old cease fire over, to which the Israelis replied that it ended with the bus bombing. Israel has said all Hamas leaders are fair game and gave Palestinians 24 hours to round up terrorists on their own before launching its campaign.

The White House reacted cautiously Friday to Israel's promise to take out the terror leaders. 

"Our focus is on getting the parties back together so they can work to resolve these issues," Bush spokesman Scott McClellan said. "We've always said that Israel has the right to defend herself. We also always pointed out that the parties, including Israel, need to keep in mind the consequence of the actions that they take — the effect of those actions on the peace process."

The Associated Press contributed to this report.