WASHINGTON --President Bill Clinton embarked on a six-nation tour of the volatile Middle East on Tuesday, vowing to stand "shoulder to shoulder" with those committed to making peace in the region.


"The time has arrived for all parties to follow the brave and hopeful inspiration of Israel and Jordan," Clinton said at the White House before heading to nearby Andrews Air Force base for the 10-hour flight to Cairo, his first stop.


Israeli authorities braced for possible attacks by Islamic militants during Clinton's visit there and his attendance at a ceremony before 5,000 guests at a desert border crossing just north of the Red Sea at which Israel and Jordan will sign a peace treaty, deploying up to 20,000 officers nationwide, according to news reports.


Police Minister Moshe Shahal told the Maariv daily it was the largest security operation in Israel since Egyptian President Anwar Sadat visited 17 years ago.


Clinton plans to attend Wednesday's treaty signing, on a plaza paved over a minefield, and on Thursday will address Israel's parliament and walk the violence-prone alleys of the Old City of Jerusalem.


Israeli police have assigned 6,000 officers for the Jerusalem stretch alone, including 500 outside his hotel, the King David.


A senior police official, speaking on condition of anonymity, told reporters Monday that the Secret Service lobbied Clinton unsuccessfully to at least cancel the Old City portion.


The United States has flown in two Blackhawk helicopters for security surveillance, and explosives detection equipment, army radio and newspapers reported.


Maariv said up to 20,000 police were being deployed nationwide.


"We have assessments that Hamas or other elements will try to carry out attacks, but we don't have any concrete warnings about time and place," National Police Chief Assaf Hefetz was quoted as telling reporters.


Hamas, a Muslim fundamentalist group, has claimed responsibility for three attacks in Israel since Oct. 9 including last week's bombing of a bus in Tel Aviv that killed 22 people.


It threatened more attacks in a leaflet published on Monday, and vowed to retaliate if Israel makes good on reported threats to assassinate its key leaders.


Graffiti painted on walls in the West Bank town of Hebron Tuesday said: "Hamas strikes with an iron hand against those who hurt it and its men."


About 100 Secret Service agents have been in Israel for the past two days arranging the president's visit, the security official said.


Police sources said the Americans were warned there would be armed Israeli agents in civilian dress in Clinton's vicinity and they should not shoot without Israeli authorization.


The president was scheduled to arrive in Cairo early Wednesday for meetings with Egyptian President Hosni Mubarak and Palestine Liberation Organization chief Yasser Arafat.


He flies to Aqabah on the Israel-Jordan border later Wednesday for the signing and then travels to Amman to address the Jordanian parliament and spend the night.


On Thursday, Clinton goes to Syria to meet with President Hafez Assad and then on to Israel to meet with Prime Minister Yitzhak Rabin, hold a joint news conference and address the Knesset.