Against a backdrop of almost universal pessimism about its chances of success, and threats by settlers to restart construction in the West Bank, the US president Barack Obama yesterday launched Washington’s third effort in a decade to end the Palestinian-Israeli conflict.

The formal resumption of direct Israeli-Palestinian negotiations after a 20-month hiatus will take place today at the US state department in Washington.

Last night Mr Obama threw his presidential authority behind the new effort by hosting separate one-to-one talks and what US officials described as a “working dinner” with the Palestinian president, Mahmoud Abbas, the Israeli prime minister Benjamin Netanyahu, King Abdullah II of Jordan, the Egyptian president Hosni Mubarak and the Quartet representative Tony Blair.

The administration has tried to strike a positive tone, insisting that a “window of opportunity” has opened to secure a two-state solution. Mr Obama’s intense involvement yesterday was meant to “build trust” between the leaders, the White House said.

Speaking at the White House last night, Mr Netanyahu called Mr Abbas “my partner in peace” and said he was seeking to end the Middle East conflict “once and for all”.

Paying tribute to the Palestinian leader, he said: “Every peace begins with leaders. President Abbas, you are my partner in peace. It is up to us to live next to one another and with one another.”

Nevertheless, there is widespread scepticism that the talks will lead to any breakthrough. Abdulrahman al Attiyah, the secretary general of the GCC, said he believed Israel had no serious intention to enter into substantial negotiations, given repeated Israeli statements that settlement construction would resume in occupied Palestinian territories.

In the West Bank yesterday, the Palestinian Authority launched an unprecedented arrest sweep in Hebron, where those responsible for Tuesday’s shooting of four Israelis are believed to be from.

As many as 300 people were detained. Of these, 150, according to Hamas, are members of the Islamist group. The military wing of Hamas, which controls the Gaza Strip and is opposed to the negotiations, claimed responsibility for the attack.

Mr Netanyahu, meanwhile, had to issue a warning to settlers not to begin construction in settlements after the Yesha Council, which represents settlers in the West Bank, had earlier pledged to start building again yesterday.

The partial and temporary freeze on settlement construction in the West Bank, which does not include occupied East Jerusalem, has been in force since November and is due to expire on September 26. On Tuesday, the Israeli leader told Hillary Clinton, the US secretary of state, that there was “no change to the ca binet decision to end the freeze at the end of September 2010”.

Palestinian officials have warned that they will walk out of talks if the freeze is not extended, a message repeated yesterday by Nabil Shaath, a senior member of the Palestinian delegation in Washington.

“We will negotiate in good faith so long as the moratorium on settlement construction continues. If Israel goes back to full settlement activities, we are out of these negotiations.”

Mr Shaath also cautioned against any efforts by Mr Netanyahu and other Israeli negotiators to exploit politically the killings of the four Israelis.

“It will have an impact if Mr Netanyahu wants it to have an impact. Because we’ve been through this before,” he said, noting other efforts to undermine talks.

“A few days ago, Rabbi Ovadia Yosef, the spiritual leader of Shas, the major partner of Mr Netanyahu, gave a sermon in which he asked God to kill all Palestinians starting with president Mahmoud Abbas, that all Palestinians should perish from the face of the earth by plague. I did not hear Mr Netanyahu make one statement condemning the rabbi’s racist incitement,” Mr Shaath said.

With the two sides far apart on most substantial issues – which include the status of refugees, borders, Jerusalem, security, settlements and water – progress will almost entirely depend on US mediation. Active US peacemaking was one of Mr Obama’s campaign promises and the president may be buoyed by having fulfilled another pledge on Tuesday, when he announced the end of combat operations in Iraq.

However, few analysts expect the president’s resolve to bear fruit in the form of a comprehensive final status agreement, which evaded his predecessors Bill Clinton in 2000 and George W Bush in 2007, or even help the Democrats as they face midterm elections on November 2.

With the Republicans already poised for large gains in Congress, Mr Obama’s high-profile peacemaking role only runs the risk of vocal right-wingers turning the issue of US support for Israel into even more of a wedge issue in election campaigns across the country.

Yet some pro-Palestinian and pro-Israeli activists working to influence US opinion in favour of a two-state solution did hold out some hope for the president’s efforts.

One factor absent in past peacemaking efforts was greater recognition of the majority of American Jews who seek an end to the conflict, said Amy Spitalnick, a spokeswoman for J Street, a liberal American-Jewish lobbying group.

She pointed to opinion polls that show Mr Obama enjoys more support from US Jews than from the general public, and most US-Jewish organisations were in agreement that this was a critical moment if a two-state solution were to emerge.

“Most American Jews understand the nuances. They support close military and strategic ties between the US and Israel as well as strong US engagement in the peace process.”

The process of energetic negotiations would help the Palestinians as they embark on building the institutions of their future state and help galvanise international opinion behind them, said Hussein Ibish, a senior fellow at the American Task Force on Palestine.

“It’s essential for talks to go ahead because they are central to Palestinian state-building and both processes could be mutually beneficial,” he said. “But if things reach a point when the talks cease to be productive, this would run the risk of discrediting the PLO, and the accreditation of Hamas.”