Barack Obama, the US president, has rejected Palestinian plans to seek statehood at the United Nations, and said Washington's commitment to Israel's security is "ironclad".
Obama was speaking at the annual policy meeting of American Israel Public Affairs Committee (AIPAC), the main pro-Israel lobby in the US, on Sunday amid unprecedented tumult in the Middle East and new strains in US-Israeli relations.
"No vote at the United Nations will ever create an independent Palestinian state," Obama said.
"And the United States will stand up against efforts to single Israel out at the UN or in any international forum. Because Israel's legitimacy is not a matter for debate."
On Thursday, the US president for the first time publicly called on Israel to accept a return to territorial lines in place before the 1967 Arab-Israeli war, with mutual land swaps with Palestinians, to frame a secure peace.
The formula was rejected by Binyamin Netanyahu, the Israeli prime minister, who told Obama at a White House summit on Friday that the 1967 borders were "indefensible".
That prompted Palestinian officials to say that they would seek recognition for Palestinian statehood in the UN General Assembly in September.
European Union states, UN diplomats say, are looking increasingly favourably on the idea of recognising a Palestinian state.
However, any statehood vote would have first to be approved in the UN Security Council, where the US - which insists on a negotiated peace accord - has a veto.
On Sunday, Obama reiterated the US's commitment to defending the existence of Israel.
"You also see our commitment to Israel's security in our steadfast opposition to any attempt to de-legitimise the State of Israel," he said.
"As I said at the United Nation's last year, Israel's existence must not be a subject for debate, and efforts to chip away at Israel's legitimacy will only be met by the unshakeable opposition of the United States."
However, Obama said the current situation in the Middle East was "unsustainable".
Obama said: "Even while we may at times disagree, as friends sometimes will, the bonds between the United States and Israel are unbreakable, and the commitment of the United States to the security of Israel is ironclad.
He also called the recent unity deal between Fatah and Hamas "an enormous obstacle to peace", and called on Hamas to release Gilad Shalit, an Israeli soldier captured five years ago.
"No country can be expected to negotiate with a terrorist organisation sworn to its destruction," Obama said.
"We will continue to demand that Hamas accept the basic responsibilities of peace: recognising Israel's right to exist, rejecting violence, and adhering to all existing agreements."
But he added: "No matter how hard it may be to start meaningful negotiations under the current circumstances, we must acknowledge that a failure to try is not an option. The status quo is unsustainable."
Rosiland Jordan, Al Jazeera's correspondent in Washington, said Obama had received a polite welcome and been applauded when he talked about the US's commitment to defending Israel's sovereignty.
But some in the 10,000 audience had booed when Obama mentioned the 1967 borders, she said.
Obama said there had been "nothing particularly original" in his speech on Thursday.
"But since questions have been raised, let me repeat what I actually said," he said.
"I said that the United States believes that negotiations should result in two states, with permanent Palestinian borders with Israel, Jordan, and Egypt, and permanent Israeli borders with Palestine.
"The borders of Israel and Palestine should be based on the 1967 lines with mutually agreed swaps, so that secure and recognised borders are established for both states.
"The Palestinian people must have the right to govern themselves, and reach their potential, in a sovereign and contiguous state."
Marwan Bishara, Al Jazeera's senior political analyst, said: "He [Obama] is saying absolutely nothing new ... he remains, I think, more or less, adamant that it has to be up to the Israelis and the Palestinians.
"Which basically, de facto, means it's up to the Israelis to dictate what exactly their security requirements are for any contours of a Palestinian solution."
Netanyahu said on Sunday that he would work alongside Obama for a way to renew frozen peace talks with
In a statement responding to Obama's speech, Netanyahu said: "I am a partner to President Obama's wish to promote peace and I appreciate his efforts in the past and present to achieve this goal. I am determined to work with Presdient Obama to find ways to renew peace talks."