President Trump told Israeli and Arab leaders on Tuesday that he plans to recognize Jerusalem as the capital of Israel, a symbolically fraught move that would upend decades of American policy and upset efforts to broker peace between Israel and the Palestinians.

Mr. Trump is expected to announce his decision on Wednesday, two days after the expiration of a deadline for him to decide whether to keep the American Embassy in Tel Aviv.

Palestinian officials said Mr. Trump told the president of the Palestinian Authority, Mahmoud Abbas, that the United States would move the embassy to Jerusalem. Jordan said the president gave a similar message to King Abdullah II.

American officials, however, said such a move could not occur immediately for logistical reasons, given the lack of facilities to house the embassy staff. As a result, Mr. Trump is expected to sign a national security waiver that would authorize the administration to keep it in Tel Aviv for an additional six months.

Still, Mr. Trump’s decision to recognize Jerusalem as the capital — and to set in motion an embassy move — is his riskiest foray yet into the thicket of Middle East diplomacy. Arab and European leaders warn that it could derail any peace initiative and even ignite fresh violence in the region.

King Abdullah II strongly cautioned against the move, “stressing that Jerusalem is the key to achieving peace and stability in the region and the world,” according to a statement from the royal palace in Amman.

“King Abdullah stressed that the adoption of this resolution will have serious implications for security and stability in the Middle East, and will undermine the efforts of the American administration to resume the peace process and fuel the feelings of Muslims and Christians,” the statement said.

Few details of the conversation between Mr. Trump and Mr. Abbas were released, but a P.L.O. spokesman said that the call had given shape to the worst fears of Palestinians — that the United States would break with decades of practice and longstanding international consensus by recognizing Jerusalem as Israel’s capital.

The Palestinians hope to make East Jerusalem the capital of a Palestinian state, and the city is of great religious significance to Jews, Christians and Muslims.

“It’s very serious,” said the spokesman, Xavier Abu Eid. “Things look very bad.”

The Palestinian news agency, WAFA, quoted Mr. Abbas’s spokesman, Nabil Abu Rudeineh, as saying that Mr. Abbas will continue his contacts with world leaders to prevent such “unacceptable action.”

King Abdullah also spoke with Mr. Abbas, assuring him of Jordan’s support for the Palestinians “in preserving their historic rights in Jerusalem and the need to work together to confront the consequences of this decision,” it said.

Mr. Trump, officials said, assured Mr. Abbas that the administration would protect Palestinian interests in any peace negotiation with Israel. He also invited the Palestinian leader to visit him in Washington for further consultations.

In his phone calls with Arab leaders, Mr. Trump is making the case that settling the question of the American Embassy could actually hasten the peace process by removing a thorny political issue that recurs every six months.

But that is primarily a political problem for Mr. Trump, who promised during the 2016 campaign to move the embassy. His pledge was extremely popular with evangelicals and pro-Israel backers, including the casino magnate Sheldon Adelson. They expressed frustration when Mr. Trump signed the waiver in June, keeping the embassy in Jerusalem.

Middle East experts said the administration’s argument that it could not move the embassy immediately made little sense, since all that is required is to place a sign on the existing American consulate, declaring it the embassy.

For Arab leaders, word that the United States would formally recognize Jerusalem as the capital had already caused great consternation. The symbolic statement of the embassy’s change of address, many officials warned, was actually less damaging to the peace process than changing United States policy on Jerusalem’s status.

For the United States to move the embassy would break with international consensus that the status of Jerusalem remains unsettled.

Though Israel houses its parliament, president, prime minister and most ministries in Jerusalem, and Israelis overwhelmingly want the world to acknowledge the Holy City as their seat of government, the international community recognizes de facto Israeli sovereignty only in West Jerusalem.

East Jerusalem was captured by Israeli forces during the Arab-Israeli war of 1967. And the permanent status of Jerusalem as a whole, East and West, was postponed under the Oslo Accords, although Israel extended Jerusalem’s municipal borders to encompass the predominantly Arab eastern neighborhoods.

Mark Landler reported from Washington and David M. Halbfinger from Jerusalem