But before beginning negotiations with Palestinian president Mahmoud Abbas in Washington, the saddened Israeli premier pledged to punish all those involved in the slaughter.

In an attempt to torpedo peace talks, Palestinian gunmen opened fire on an Israeli car in the volatile West Bank city on Tuesday and killed four two men and two women - one of them pregnant.

The victims were Yitzhak Ames, 47, and his wife Tali Ames, 45, Kochava Even-Haim, 37, and Avishai Schindler, 24, all from the settlement of Beit Hagai.

Mr and Mrs Ames had six children, including an 18-month-old toddler. The couple were buried on Wednesdat on Jerusalem's Mount of Olives. Even Haim was buried in Ashdod and Schindler in Petach Tikva.

The car was riddled with bullets, giving the passengers and driver no chance.

The attack took place near Bani Naim after dark on Highway 60 - a busy route used by both Palestinians and Jewish settlers.

According to eyewitness reports, the terrorists succeeded in hitting the passengers in their initial fire, but then approached the car and shot the occupants again at close range.

Netanyahu was given the news several hours into his flight from Tel Aviv to the United States.

The mood among the Israeli officials and press corps changed visibly and the grief was etched on the face of the prime minister when he stepped on to the tarmac in Washington.

Secretary of State Hillary Clinton also looked forlorn as she later met the Israeli leader in a Washington hotel.

He told her that this "criminal attack" proves again the need for Israel to stand steadfastly by its security demands.

Netanyahu said that his heart went out to the families of the victims, and that terrorism would not determine Israel's borders or the future of the settlements.

Clinton said: "We pledge to do all we can always to protect and defend the State of Israel and to provide security to the Israeli people."

Palestinian leader Abbas told the world's Press that the attack was designed to disrupt the peace process.

"It can't be regarded as an act of resistance." he added.

Abbas said that he condemned all acts that target Palestinian and Israeli civilians.

A statement from the White House said: "On the eve of the re-launch of direct negotiations, this brutal attack underscores how far the enemies of peace will go to try to block progress.

"It is crucial that the parties persevere, keep moving forward even through difficult times, and continue working to achieve a just and lasting peace in the region that provides security for all peoples."

The military wing of Hamas later said it had carried out this "heroic operation".

Palestinian security forces immediately rounded up more than 200 Hamas-affiliated suspects in the area

To mark the relaunch of direct peace talks, President Barack Obama hosted a working dinner on Wednesday with all visiting leaders, Mrs Clinton and UN envoy Tony Blair.

Before leaving the West Bank, Abbas urged Israel "not to miss this historic opportunity for peace".

He added: "If there is only a one per cent chance of achieving peace we will strive for it".

And Netanyahu told reporters before leaving Ben Gurion airport: "I am hoping for a peace based on recognition, security, stability and economic prosperity between the two peoples that will endure for us and our children".