ERUSALEM, March 14 — In announcing today that he was prepared to move ahead with a "road map" to peace and a Palestinian state, President Bush appeared to diverge sharply from the allies who helped him draft the map over what, precisely, it represents.

Mr. Bush pleased Israelis and dismayed Palestinians by describing the draft proposal as open to amendment, saying, "We will expect and welcome contributions from Israel and the Palestinians to this document that will advance true peace."

The three other members of the diplomatic quartet that drew up the plan — the United Nations, the European Union and Russia — regard it as fixed, demanding immediate concessions from both sides, according to diplomats involved in the process. Israel has criticized it as potentially threatening to its security and has sought many changes.

Even as he made his announcement, Mr. Bush altered the document. He said he would present it as soon as the Palestinians confirmed a prime minister with "real authority."

Mr. Bush's intention may be to box in Yasir Arafat, the Palestinian leader, and force him to install a powerful prime minister. But according to the plan, which the quartet agreed on in December, the prime minister is supposed to be appointed as part of the first phase, which also demands difficult steps from Israel.

After postponing action on the plan for months, Mr. Bush has chosen to act at a moment of some diplomatic possibility and great American leverage here. Mr. Arafat has begun to move on a prime minister, while Ariel Sharon of Israel has said the only way out of Israel's deep recession is an end to the conflict. Israeli officials also are seeking a multibillion-dollar emergency aid package from Washington.

Mr. Bush's wording was far less precise than that of the plan itself. He may be trying to remain ambiguous enough to create room to maneuver for Mr. Sharon, whose rightist government rejects key aspects of the plan. If so, that is a gamble. Over the last two years, other plans to restart talks have collapsed in negotiations over exactly what the wording of the documents meant.

Mr. Bush's remarks caught at least some of the administration's diplomatic allies off guard and left them wondering how conflicting Israeli and Palestinian changes might be reconciled.

"It's not meant to be a negotiated document," one Western diplomat said. He said other members of the quartet would construct their own interpretation of Mr. Bush's comments. "We will understand President Bush to mean, when he says `contributions,' `additional details to be added,' " rather than changes to the existing plan, this diplomat said.

But Israeli officials interpreted Mr. Bush's remarks more broadly. Prime Minister Sharon has said he accepts the plan, provided that it strictly fulfills the terms of a speech delivered by President Bush last June 24. Compared with the plan, that June speech was interpreted by both sides as placing more burdens on the Palestinians in the short term.

Israeli officials said they heard nothing tonight to conflict with that approach. Gideon Meir, deputy director general of the Foreign Ministry, said any plan that would "reflect precisely the presidential vision will be an important tool to implement the speech" of June 24.

He said a Palestinian prime minister who was "totally disconnected from Arafat" and who would be "acting decisively against terror and incitement" and rebuilding the governing Palestinian Authority would be a partner who "together with Israel will give its response to the road map."

Saeb Erekat, the Palestinians' chief negotiator, was clearly alarmed, saying, "If we're going to introduce the road map for discussions, it means at the end of the discussions there will be no road map."

Palestinian officials have said that like the Israelis, they dispute aspects of the plan but accept it as a whole in the belief that it is to be imposed on both sides.

Prime Minister Tony Blair of Britain has pushed for President Bush to announce the plan, but even Mr. Blair seemed to have a different idea than his ally of what the plan stands for. He did not emphasize possible changes, but instead spoke of "specific steps that we are committed to." He said Israel was expected to institute "a freeze on all settlement activity" as part of the first phase.

Mr. Bush offered a more elastic formula on settlements. "As progress is made toward peace, settlement activity in the occupied territories must end," he said.

Mr. Sharon, an architect of the settlement movement, has built a governing coalition that includes two parties closely identified with settlers, as is his own faction, Likud. He would almost surely have to form a new government with leftist factions to sustain a major move to restrain settlement.

At issue in the dispute over the plan is a fundamental divide, a seemingly arcane difference over process. It is referred to here as a debate between "sequentialism" and "parallelism" — over whether one side must make concessions before the other side acts.

Mr. Sharon has demanded that the Palestinians first meet a series of conditions, including halting all violence and dismantling all terrorist groups, before a return to negotiations. President Bush has largely supported his conditions.

But the plan outlines a parallel process of simultaneous concessions. After hearing Mr. Bush today, a Western diplomat said, "He's using the vocabulary of sequentialism."