Barack Obama has told the UN General Assembly the Palestinians deserve their own state, but that this would only be achieved through talks with Israel.
The US president's speech came as diplomatic efforts for Palestinian UN membership intensified, while thousands rallied in the West Bank.
"Peace will not come through statements and resolutions at the UN," he said.
He added there could be no "short cut" to peace, and is expected to urge the Palestinians to give up the initiative.
Mr Obama is holding talks with Israeli Prime Minister Benjamin Netanyahu, and is to meet Palestinian leader Mahmoud Abbas later.
Mr Abbas is set to launch the statehood bid on Friday, after his address to the UN General Assembly, with a written request to Secretary General Ban Ki-moon.
If his request is approved by Mr Ban, the Security Council will then examine and vote on it. In order to pass, the request must get the votes of nine out of 15 council members, with no vetoes from the permanent members.
The US has vowed to veto the request, and correspondents say Western diplomats are exploring ways to put off the voting process to buy themselves more time.
Final status issues
"I am convinced that there is no short cut to the end of a conflict that has endured for decades," Mr Obama told the UN summit in New York.
"Ultimately, it is Israelis and Palestinians - not us - who must reach agreement on the issues that divide them: on borders and security; on refugees and Jerusalem."
Susan Rice, the US ambassador to the UN, called the Palestinian bid "an unwise and diversionary gambit", adding: "There is no shortcut to statehood."
UN Secretary General Ban Ki-moon opened the summit with a call for new international efforts to break the Middle East "stalemate".
He pledged "unrelenting" UN efforts to push forward the peace process.
While UN recognition would have largely symbolic value, the Palestinians argue it would strengthen their hand in peace talks.
In the West Bank on Wednesday, schools and government offices were shut to allow for demonstrations backing the UN membership bid in Ramallah, Bethlehem, Nablus and Hebron.
'Time has come'
In his meeting with Mr Obama, Mr Netanyahu said direct negotiation was the only way to achieve a stable Middle East peace, adding that the Palestinian statehood effort was itself a short cut that "will not succeed".
Israeli President Shimon Peres said earlier that peace could only be achieved if the security of Israel was assured.
"The United Nations cannot guarantee the security of Israel," he said.
But Palestinians say their bid for statehood has been inspired by the Arab Spring, and is the result of years of failed peace talks.
"The end of the Israeli occupation and a Palestinian state are the only path to peace," AFP quoted Mr Abbas' spokesman as saying after Mr Obama's speech.
"We will agree to return to the negotiations the minute that Israel agrees to end the settlements and the lines of 1967," added the spokesman, Nabil Abu Rudeina.
The last round of talks broke down a year ago.
Efforts are now reportedly under way for the "quartet" of US, European, Russian and UN mediators to provide a basis for resumed peace negotiations.
But work by mediators has yet to produce guidelines for the resumption of talks.
The quartet aims to give the two sides a year to reach a framework agreement, based on Mr Obama's vision of borders fashioned from Israel's pre-1967 boundary, with agreed land swaps.
Both Mr Netanyahu and Mr Abbas have said they are prepared to engage in direct talks.
But Mr Abbas has so far appeared determined to press ahead with the statehood bid.
Should the Palestinian effort at the Security Council fail, Mr Abbas could ask for a vote of the General Assembly for enhanced observer status - which is enjoyed by others such as the Vatican - in which case no veto would be possible.
9/21/11
Obama addresses UN as Palestinian bid prompts rallies
11:46 PM
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