"Israel's plan to annex the Jordan Valley, an integral part of occupied Palestine, is manifestly illegal and merely adds to Israel's long history of violations of global law", Erekat, a former peace negotiator, said in a statement.
About 450,000 Israeli citizens live in settlements, considered illegal by most of the worldwide community, on territory that Israel has ruled since its victory in the 1967 Israeli-Arab war.
Israeli Prime Minister Benjamin Netanyahu speaks at a news conference in Tel Aviv on September 10, 2019.
Netanyahu said his annexation plans would not include Palestinian cities, such as the Jordan Valley's Jericho, though it would be encircled by Israeli territory.
Palestinian official Saeb Erekat said Netanyahu's annexation plan is "manifestly illegal".
"Out of respect for President Trump and great faith in our friendship, I will await applying sovereignty until release of the president's political plan", he said, referring to a long-awaited blueprint from Washington for Israeli-Palestinian peace.
Gantz also said that "the relationship between Israel and the United States is founded upon common interests and values, and it is stronger than any prime minister".
Ahead of the announcement, Palestinian Prime Minister Mohammad Shtayyeh warned against such a move, saying that "the Palestinian territory is not part of Netanyahu's election campaign".
A few days before the April legislative elections, which failed to produce a government, Netanyahu had already promised to annex Jewish settlements in the occupied West Bank.
He added that the U.S. president was likely to release his long-touted Middle East peace plan soon.
White House senior adviser Jared Kushner said in early May that he hoped Israel would take a hard look at President Donald Trump's upcoming Middle East peace proposal before "proceeding with any plan" to annex West Bank settlements. Unlike Israeli settlers, West Bank Palestinians are not Israeli citizens and do not have the right to vote.
The U.S. plan, Netanyahu reiterated, would likely be presented very soon after Israel goes to the polls on September 17.
Ben-Ami encouraged US House leadership to pass a resolution that unequivocally opposes unilateral Israeli annexation of any territory within the West Bank and reaffirms its support for a two-state outcome to the conflict.
"We haven't had such an opportunity since the Six Day War and I doubt we will have another opportunity in the next 50 years".
Ahead of his speech, Netanyahu's team billed the announcement as "dramatic" and allowed rumors to float about what the troubled leader would be discussing just one week ahead of his next election. Unlike East Jerusalem and the Golan Heights, Israel never extended its sovereignty to the West Bank.
The changes, however, have all but eliminated the idea of a peaceful agreement between Palestine and Israel.