WASHINGTON -- President Barack Obama Wednesday opened long-odds Middle East talks, as Israeli Prime Minister Benjamin Netanyahu vowed to defy skeptics by forging a "once and for all" peace with the Palestinians.

Obama warned Hamas meanwhile that its "senseless slaughter" of four Israelis in the West Bank would not thwart his efforts, while slim hopes were tempered by persistent tension between Israelis and Palestinians over settlements.

The US president, wagering substantial political capital, plunged into the kind of personal peacemaking effort which ended in frustration for his recent predecessors, bringing five key power players to the White House.

He was separately meeting Netanyahu, Palestinian president Mahmud Abbas, Jordan's King Abdullah II and Egyptian President Hosni Mubarak, hoping to build trust, on the eve of the first Israeli-Palestinian direct talks in 20 months.

Netanyahu, who heads a conservative coalition which some observers believe will preclude concessions, insisted he was committed to a good faith effort.

"Our goal is to forge a secure and durable peace between Israel and the Palestinians," Netanyahu said in an advance copy of his remarks at a formal joint White House appearance of the five leaders.

"We seek a peace that will end the conflict between us once and for all. We seek a peace that will last for generations.

"President Abbas, you are my partner in peace. It is up to us to live next to one another and with one another."

But Netanyahu also included a tough warning that he would balk at any deal that did not assuage Israel's security concerns.

"We left Lebanon, we got terror. We left Gaza, we got terror. We want to ensure that territory we concede will not be turned into a third Iranian sponsored terror enclave aimed at the heart of Israel."

In another reminder of the fractious regional situation, Obama was forced to respond to a bid by Hamas to kill his initiative, after the Islamist movement gunned down four Israelis, including a pregnant woman, in the West Bank.

"The tragedy we saw yesterday, by terrorists who are purposely trying to undermine these talks, is an example of what we are up against," Obama said, with Netanyahu at his side.

Obama vowed to "push back" against "terrorist activities."

The direct talks on Thursday -- the result of painstaking US diplomacy -- will take place with few of the parties, or outside observers, predicting success amid widespread regional distrust.

But Obama said after meeting Abbas that he was making "progress" -- though there was little sign Israelis or Palestinians had modified entrenched positions.

Netanyahu told US Secretary of State Hillary Clinton late Tuesday there was no change to the Israeli position that a partial freeze on settlement building on the West Bank expires at the end of September, his office said.

But the Palestinians immediately warned a resumption of settlement construction would mean the end of peace talks.

"The settlements must be halted and continuing them will signal the end of the peace process," Palestinian spokesman Nabil Abu Rudeina told journalists.

Hamas, which rules Gaza, is opposed to the peace talks and is a rival of Abbas's US-backed Palestinian Authority.

The issues on the table at the US-mediated talks -- the status of Jerusalem, security, the borders of a Palestinian state and the right of return for Palestinian refugees have confounded all previous mediation attempts.

But the White House insists that a "window of opportunity" has opened up to forge a two-state solution in the Middle East, at a time when Iran's growing influence is threatening to reset the regional political map.

The Quartet of the United States, the European Union, the United Nations and Russia launched a roadmap for peace in 2003 that calls for a Palestinian state living alongside a secure Israel.

Top level talks in search of the elusive peace deal broke off in 2008 when Israel invaded the Palestinian Gaza Strip to halt militant rocket fire on its south.