On Tuesday Mr Netanyahu pledged to apply Israeli sovereignty over the Jordan Valley and northern Dead Sea if he is returned to office following a general election next week.
Officials in Jordan and Saudi Arabia, as well as in Turkey, sharply criticised the announcement.
The chief Palestinian negotiator said the move would be "manifestly illegal".
The annexation proposed by Mr Netanyahu would "bury any chance of peace", Saeb Erekat said.
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Palestinian territories profile
Israel has occupied the West Bank since 1967 but has stopped short of annexation.
Palestinians claim the whole of the area for a future independent state. Mr Netanyahu has previously insisted Israel would always retain a presence in the Jordan Valley for security purposes.
A UN spokesman said the proposed annexation would have "no international legal effect".
What did Netanyahu say?
The Israeli PM, whose right-wing Likud party is neck and neck in the polls with an opposition centrist alliance, unveiled the plan in a televised address.
"Today, I announce my intention, after the establishment of a new government, to apply Israeli sovereignty to the Jordan Valley and the northern Dead Sea," Mr Netanyahu said.
He said this could be done immediately after the election if he received a "clear mandate" from Israeli citizens.
He said the move would allow Israel to seize a historic opportunity, but his domestic political opponents dismissed the pledge as an election stunt.
He also again pledged that he would annex all Jewish settlements in the West Bank, but said this would need to wait until the publication of US President Donald Trump's long-awaited plan for a peace agreement between Israelis and Palestinians.
Mr Netanyahu first made that pledge ahead of an election in April. But after the vote he failed to successfully form a workable coalition government, paving the way for next Tuesday's snap election.
The BBC's Middle East Correspondent, Tom Bateman, says the prime minister's announcement is likely to help him shore up support on the political right.
Blue and White's co-leader, Yair Lapid, hit out at Mr Netanyahu, insisting he "doesn't want to annex territories, he wants to annex votes".
"This is an election trick and it's not even a particularly successful trick because the lie is so transparent," he said.
Meanwhile White House officials said that US policy remained unchanged.
Later on Tuesday, Mr Netanyahu was whisked off stage by bodyguards during a campaign event in Ashdod, a southern city, as air raid sirens went off.
Israel said it shot down two rockets that had been fired by Palestinians from the Gaza Strip. Mr Netanyahu continued his speech several minutes later.
In response, Israel said early on Wednesday that its aircraft had hit 15 targets in Gaza, including a weapons facility.
How have others responded?
A spokesman for UN Secretary General Antonio Guterres said the move would be "devastating" to the prospects of revived peace negotiations.
"Any Israeli decision to impose its laws, jurisdictions and administration in the occupied West Bank is without any international legal effect," Stéphane Dujarric added.
The Arab League - an organisation of 22 states - said Mr Netanyahu's plan was a "dangerous development" that would violate international law and "torpedo" the foundations of peace.
Senior Palestinian official Hanan Ashrawi said the plan was "unquestionably reprehensible" and a "threat to international peace and security".
"This announcement is a declaration of war against the Palestinian people's rights as well as the very foundations of the international rule-based order," she said.
Jordan's Foreign Minister, Ayman Safadi, called the plan a "serious escalation" and warned it could "push the whole region towards violence".
Turkey's Foreign Minister, Mevlut Cavusoglu, described the pledge as "racist" and criticised Mr Netanyahu for "giving all kind of illegal, unlawful and aggressive messages" before the election.
Saudi Arabia also condemned the announcement on state media as a "very dangerous escalation" and called for an emergency meeting of the foreign ministers of the 57 member states of the Organisation of Islamic Cooperation (OIC) in response.
What is the background to the West Bank issue?
Israel occupied the West Bank, along with East Jerusalem, Gaza and the Syrian Golan Heights, in the 1967 Middle East war. It effectively annexed East Jerusalem in 1980, and the Golan Heights in 1981, although neither move was accepted internationally for decades.