Israel's Prime Minister Benjamin Netanyahu has vowed to to annex the Jordan Valley in the occupied West Bank if re-elected.

"There is one place where we can apply Israeli sovereignty immediately after the elections," Mr Netanyahu said in a televised speech.

"If I receive from you, citizens of Israel, a clear mandate to do so ... today I announce my intention to apply with the formation of the next government Israeli sovereignty over the Jordan Valley and northern Dead Sea."

But Mr Netanyahu said such a move would not be made before publication of a long-awaited US peace plan and consultations with President Donald Trump.

A senior Palestinian official said the pledge to annex the Jordan Valley destroyed all chances of Israeli-Palestinian peace.

"He is not only destroying the two-state solution, he is destroying all chances of peace," Hanan Ashrawi said.

She added that the move, which would leave the Palestinian government with self-rule over a number of cities in the West Bank, was "worse than apartheid."

"He is trying to take the land without the people and saying you are free to leave," she said.

Palestinian Islamist movement Hamas accused Mr Netanyahu of seeking "right-wing votes by selling the illusion to his public that he can occupy Palestinian land forever."

"In every (Israeli) election we pay the price, with our rights, with our lands."

Ahead of Mr Netanyahu's announcement, Palestinian Prime Minister Mohammad Shtayyeh, issued a statement amid reports of a possible annexation announcement, said that the Israeli leader is "a prime destroyer of the peace process".

More than 400,000 Israelis live in West Bank settlements considered illegal under international law. Around 2.7 million Palestinians live in the territory.

The United Nations warned Mr Netanyahu that his plan to annex the Jordan Valley in the occupied West Bank would have no "international legal effect".

"The secretary-general's position has always been clear: Unilateral actions are not helpful in the peace process," UN spokesman Stephane Dujarric said.

"Any Israeli decision to impose its laws, jurisdictions and administration in the occupied West Bank is without any international legal effect," the spokesman added.

"Such a prospect would be devastating to the potential of reviving negotiations, regional peace, and the very essence of a two-state solution."

Israeli settlements are located in what is known as Area C of the West Bank, which accounts for some 60 percent of the territory, including most of the Jordan Valley.

The Jordan Valley accounts for around one-third of the West Bank and Israeli right-wing politicians have long viewed the strategic area as a part of the territory they would never retreat from.

Israel captured the West Bank in a 1967 war.

The election will take place next Tuesday 17 September.

Mr Netanyahu, who became the country's longest serving prime minister in July, is seeking a record fifth term in office.

This election is the second to be held this year after he failed to form a coalition government in May.