JERUSALEM — Ehud Olmert pledged Thursday to immediately resign as Israel's prime minister once his party chooses his successor as leader next week, shooting down speculation he would try to hold on to his office.

The announcement means Israel could find itself racing to form a new government in as little as a seven days. And it raises new questions on Washington's stated goal of brokering an Israeli-Palestinian peace agreement by year's end.

Olmert, who is battling a corruption investigation, announced in July that he would resign after his Kadima Party chooses a new leader in Wednesday's primary. But some felt he was vague about the timing of his exit, raising speculation he would try to keep power.

Olmert said Thursday he never intended to delay his resignation.

"As I have said before, immediately after the selection of a new chairman of Kadima, I plan to resign and recommend to the president to pick the new head of the party to form a government," he said.

However, under Israel's complicated political system, Olmert could find himself in office well into next year, even if that is not his intention.

He would stay on as a caretaker while the new Kadima leader tried to form a ruling alliance. If that failed, Israel would have to hold elections a year and a half early. Coalition negotiations after a ballot could last until spring, and Olmert would remain in office until then.

Though Kadima holds its primary Wednesday, it could be forced to hold a second round the following week if none of the four contenders gets 40 percent of the vote.

Foreign Minister Tzipi Livni, Israel's lead negotiator in peace talks, and Cabinet minister Shaul Mofaz, a former defense minister and military chief, are the front-runners.

Opinion polls, meanwhile, indicate a tight race in any national ballot between Livni and hard-line former Prime Minister Benjamin Netanyahu, whose election could set back the U.S.-sponsored Mideast peace drive.

With that in mind, both Kadima front-runners are expected to try to keep the current governing coalition intact. They might not succeed, however, because coalition partners are likely to push new demands for funds for pet programs or plum Cabinet posts.

During the transition, Olmert would find it difficult to push forward with his major diplomatic initiatives — peace talks with the Palestinians and separate, Turkish-mediated negotiations with Syria.

If elected, Livni, who hopes to become Israel's first female prime minister in four decades, would likely push ahead with the peace talks she has been leading. Mofaz would be expected to take a tougher line, particularly on the sensitive issue of Jerusalem. He opposes sharing control of the city, whose eastern sector is sought by the Palestinians as a future capital.

Police have been investigating for months a string of corruption cases involving Olmert. In one, he is suspected of improperly accepting cash and fancy gifts from a U.S. supporter. In another, he is suspected of submitting multiple bills for trips abroad, pocketing the difference or financing trips for relatives.

All the cases date to before Olmert became prime minister in 2006. Olmert, who says he is innocent, has not been charged, though police investigators this week formally recommended he be indicted on bribery, breach of trust and money laundering charges.

Also on Thursday, an Israeli human rights group said Jewish settlers have seized thousands of acres on the West Bank by widening their communities' perimeter fences or scaring Palestinian farmers off their fields through harassment.

The report by B'Tselem said 12 settlements had seized 1,100 acres, more than doubling the area under their control, by pushing out their fences with government backing since 2002.

Other settlements have gradually taken thousands of acres since the 1970s by building extended fences without permission or by expelling Palestinians from areas near settlements, the group said.

Yigal Palmor, spokesman for the Israeli Foreign Ministry, said that any fences built in the West Bank were temporary security measures and that the final status of the territory would be determined in the peace talks.