US President George W. Bush heads to the Middle East on Tuesday, fresh from warning Syria and Iran over unrest in Lebanon and reaffirming optimism about embattled Israeli-Palestinian peace efforts.

Renewed turmoil in the region threatened to derail any significant progress in long-stalled talks, including a corruption scandal surrounding Israel's prime minister, violence in nearby Lebanon and defiance from Hamas over conditions for a Gaza truce.

Bush departs at 5:30 pm (2130 GMT) on the five-day trip with stops in Israel, which is marking its 60th anniversary, as well as Saudi Arabia to mark 75 years of US relations with the kingdom, and talks in Egypt with a broad range of regional leaders.

The visit is Bush's second in four months -- after seven years in which he did not set foot in either Israel or the Palestinian territories -- amid hopes a peace deal will shore up his legacy with just over eight months left in his term.

"The vision of a (Palestinian) state is such a powerful notion and such an important notion for Israel's very existence, that I do believe that we have a chance to get something defined," Bush told reporters on the eve of the trip.

However, he added he "will come not as somebody who demands, but somebody who encourages."

"I'm not running for the Nobel Peace Prize. I'm just trying to be a guy to use the influence of the United States to move the process along," Bush said.

Israel and the Palestinians relaunched formal peace negotiations in November at a US conference hosted by Bush, who said he hoped to see the signing of a full peace agreement before leaving office in January 2009.

However, senior Israeli officials described the event as Bush's "farewell visit" and expected little substance.

Bush is to meet on Wednesday with Israeli Prime Minister Ehud Olmert, who is embroiled in allegations he took bribes from a millionaire US financier and faces mounting calls to resign.

Bush is to be accompanied by Secretary of State Condoleezza Rice, who put the onus on regional leaders when asked on CBS Tuesday if eight months was enough time for a deal to be brokered.

"If the parties want to do this and will put the energy and the will into it, I think they can do it," she said, adding she was "impressed by their commitment (and) seriousness."

"I'm also a big believer that nothing's really impossible. It might be improbable, but it's not impossible."

Palestinians are concerned that the Olmert affair could make Israel take a harder line regarding settlement building, lead to military escalation and further stall any peace talks.

Since the November agreement between Olmert and Palestinian president Mahmud Abbas, Israel has announced plans to pursue construction in Jewish settlements in occupied Palestinian lands -- a core dispute. And the United States has said that its ally has not done enough to improve Palestinian quality of life.

Another major obstacle is Hamas's control of the Gaza Strip, launch site for frequent rocket or mortar attacks on Israel, which the Palestinian militant group does not recognize.

As Palestinians marked the 60th anniversary of what Palestinians call the Naqba, or catastrophe -- the mass exodus of refugees at the creation of the Jewish state -- Hamas warned it had no intention of agreeing to Israel's demand it release an Israeli soldier held captive nearly two years in exchange for a Gaza truce.

Bush has no plans to visit the Palestinian territories this time, nor to hold a joint summit with Olmert and Abbas -- though he will see the Palestinian leader in Egypt May 17.

Bush faces another crisis in Lebanon, where Israel foe Hezbollah -- the two fought a war in 2006 -- has led in recent days an armed campaign against forces loyal to the pro-Western government which has resulted in more than 60 deaths.

Bush blasted Hezbollah's "recent efforts, and those of their foreign sponsors in Tehran and Damascus, to use violence and intimidation to bend the government and people of Lebanon to their will," and vowed to help by strengthening the Lebanese armed forces.

On May 16, Bush visits Saudi Arabia, which he hopes will help push the peace process but also play a larger role in stabilizing Iraq. He was also to express US concerns about the dramatic rise in oil prices.