The killing of four Israeli settlers, including a pregnant woman, in the West Bank on Tuesday evening rattled Israeli and Palestinian leaders on the eve of peace talks in Washington and underscored the disruptive role that the issue of Jewish settlements could play in the already fragile negotiations.

The military wing of Hamas, the Islamic group, claimed responsibility for the attack — in which gunmen fired on a vehicle carrying two men and two women at a junction near the city of Hebron — and described it on its Arabic Web site as a “heroic operation.”

Prime Minister Benjamin Netanyahu condemned the “atrocious murder,” which Israeli officials said seemed calculated by Hamas to upset the negotiations, which it virulently opposes. Mr. Netanyahu, en route to Washington at the time, said, “Terror will not determine the borders of Israel or the future of settlements.” He ordered Israel’s security forces “to pursue the attackers without any diplomatic restraint,” his office said.

The Palestinian Authority also condemned the attacks, which occurred just before its president, Mahmoud Abbas, met with Secretary of State Hillary Rodham Clinton. A Palestinian spokesman, Nabil Abu Rudeineh, said the attack by Hamas, the authority’s rival, underlined “the need to proceed quickly toward a just and lasting peace agreement,” which he said would “put an end to these acts.”

Even before the attack, settlements were looming as a potential deal-breaker in the peace process. Mr. Netanyahu has steadfastly refused to commit to extending a partial moratorium on construction in the West Bank, which expires Sept. 26, while Mr. Abbas has said it will be very hard to keep talking if construction resumes. Mr. Netanyahu has not struck any private deals with President Obama or anyone else on the moratorium, American and Israeli officials said.

Still, the Obama administration, according to officials, is calculating that once the two leaders are in face-to-face negotiations, neither side will be willing to take actions that would capsize the talks in the first month. Mr. Netanyahu, this thinking goes, will offer a compromise that, while it may fall short of an extension of the moratorium, will satisfy the Palestinians that construction will be curbed.

The White House said in a statement: “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.”

Mr. Netanyahu and Mr. Abbas are scheduled to meet Mr. Obama for dinner on Wednesday at the White House, and formally begin negotiations on Thursday in a meeting with Mrs. Clinton. If that goes well, officials hope for a second meeting, possibly in Egypt, in mid-September. Mrs. Clinton may take part in that session, officials said.

A senior Israeli official said that the West Bank attack, the deadliest on Israeli citizens in more than two years, would inevitably heighten the emphasis on Israel’s security in the negotiations. But Palestinian officials noted that the attack took place in an area of the West Bank that is under full Israeli security control, and where the Palestinian security forces have no responsibility and are not allowed to operate.

The victims came from Beit Hagai, a small settlement in the hills south of Hebron, an area known for particularly militant settlers. Israeli forces were combing the area looking for suspects.

Salam Fayyad, the prime minister of the Palestinian Authority, said in a statement, “We condemn this operation, which contradicts Palestinian interests and the efforts of the Palestinian leadership to garner international support for the national rights of our people.”

Hamas controls Gaza, while Mr. Abbas’s authority is limited to governing the Palestinians of the West Bank.

The claim of responsibility was unusual in that Hamas had refrained from taking responsibility for attacks in recent years. But with a sharp decline in rocket fire from Gaza against southern Israel, some in Gaza have been questioning Hamas’s commitment to fighting Israel.

Hamas said the attack was “a natural response to the crimes of the Israeli occupation and its settlers.” It said the killings demonstrated that the “armed Palestinian resistance is present in the West Bank despite the war to uproot it,” referring to the crackdown by Israeli and Palestinian Authority forces against Hamas in the West Bank.

In the Jabaliya refugee camp in northern Gaza, hundreds of Hamas supporters took to the streets after the evening prayer to celebrate the news of the attack, urged on by the calls of an imam over the loudspeaker even before Hamas had officially said it was behind the killings.

Mushir al-Masri, a Hamas lawmaker, criticized Mr. Abbas during the rally, saying he was going to the negotiations “representing only himself.” The attack “was the Palestinian people’s response to the talks,” Mr. Masri said.

Lt. Col. Avital Leibovich, an Israeli military spokeswoman, said the shooting followed a two-year period of stability in the West Bank, during which Israel eased many restrictions on the movement of Palestinians. Roadblocks in the area of the attack were removed two years ago.

Settlers, many of whom have little faith in the Palestinians or the prospects of peace, were enraged by the attack. Tzviki Bar-Hai, the chairman of the South Mount Hebron settlers’ council, told Israel Radio, “For the past 100 years there has been a link between the Jewish people’s desire to live and the Arab people’s desire to kill us.”

In July, Israeli security officials said they had arrested several members of Hamas’s military wing who were responsible for the fatal shooting of an Israeli police officer south of Hebron in June.

The stop-and-go Israeli-Palestinian peace process has often taken place in the shadow of bloody attacks. Yitzhak Rabin, the Israeli prime minister who led the Oslo peace process in the early and mid-1990s, said his philosophy was “to fight terror as if there were no negotiations and conduct the negotiations as if there was no terror.” Mr. Rabin was assassinated by an Israeli right-wing extremist in 1995.