U.S. President George Bush will warn that letting Iran obtain nuclear weapons would be - in his words - an "unforgivable betrayal of future generations."
In remarks prepared for delivery Thursday to Israel's parliament, the Knesset, Mr. Bush says for the sake of peace, the world must not allow Iran to have a nuclear weapon.
He calls Iran the world's leading sponsor of terror.
President Bush also says the United States and Israel have an "unbreakable" alliance. He rejects the argument that if Washington breaks ties with the Jewish state, then all of its problems in the Middle East will go away.
Mr. Bush says Islamic extremist groups al-Qaida, Hezbollah and Hamas will be defeated, as Muslims across the Middle East - as he puts it - "recognize the injustice of their cause."
Mr. Bush is also expected to issue a call for Israelis and Palestinians to overcome their differences and reach a peace deal by the end of the year.
Before addressing Israel's parliament, Mr. Bush visited Masada, the Roman-era fortress that overlooks the Dead Sea. Masada is believed to be where Jewish rebels killed themselves 2,000 years ago rather than fall into slavery under the Romans.
From Israel, Mr. Bush heads to Saudi Arabia, where he said the issue of world oil supplies is sure to arise in talks with King Abdullah.
The U.S. president also will travel to Egypt for meetings with President Hosni Mubarak, Palestinian President Mahmoud Abbas and Jordanian King Abdullah.