President Bush forcefully defended his Middle East policy Monday and challenged Israeli and Palestinian leaders to consistently show the will to end the violence.

Bush also called on European and Arab nations to take a more aggressive role in the peace process.

Of Palestinian leader Yasser Arafat, Bush said, "I think he can do a lot more to be convincing the people on the streets to stop these acts of terrorism and acts of violence.

"It is very important for Mr. Arafat to show 100 percent effort, do everything he can, to convince the different parties on the West Bank and in Gaza to stop the violence."

Bush sidestepped a question about whether he will invite Arafat to Washington, which he thus far has declined to do.

"I will invite the respective parties to come see me at the appropriate time," he said.

Bush spoke a day after a suicide bomber injured 20 people in a suburb of the northern Israeli port city of Haifa. The 28-year-old assailant was killed.

Bush deplored such terrorist acts as despicable.

"We recognize there could be isolated incidences of terror," he said, "but these aren't isolated. This is a continuing terror campaign."

Critics in the Middle East and at home have questioned whether Bush is working vigorously enough to help achieve a peace pact in the region. Bush, seeking to answer those critics, said: "We have been engaged in the Middle East ever since I got sworn in." He noted that he had dispatched CIA Director George Tenet to the region.

Tenet negotiated a cease-fire earlier this summer, but it unraveled in recent days.

Bush, who is on vacation at his ranch, said he had not spoken to Middle East leaders in the last couple of days but that Secretary of State Colin Powell had. He said he knew the nature of the conversations but did not disclose it.

His administration was on the phone almost on a daily basis to the respective parties, Bush said.

Bush repeatedly asserted his desire to help bring peace but each time said the primary responsibility rests with the Israelis and Palestinians.

"It's so important for there to be the will, the desire," he said. "It requires two parties to make the conscious decision that we're going to do everything we can to stop terrorism.

"Sometimes we see the will on the other side and sometimes that cycle overcomes the will," he said.