Standing next to Mr. Tillerson, the European Union’s top diplomat, Federica Mogherini, made clear that Europe saw the president’s announcement as a threat to peace in the Middle East.
“We believe that any action that would undermine these efforts must absolutely be avoided,” she said. “A way must be found through negotiations to resolve the status of Jerusalem as a future capital of both states.”
Just hours before Mr. Trump was expected to declare Jerusalem the capital of Israel, Mr. Tillerson said that peace in the Middle East was still possible.
Mr. Tillerson, during a news conference at NATO headquarters in the heart of Europe, said that he did not want to pre-empt the president’s official announcement, but expressed reassurances about the expected consequences of the decision.
“The president’s very committed to the Middle East peace process,” Mr. Tillerson said.
Mr. Tillerson has been largely shut out of the usual back-and-forth between Israelis and Palestinians that many secretaries of state spent much of their tenures conducting. Instead, Mr. Trump entrusted that task to his son-in-law and senior adviser, Jared Kushner.
At least one former Obama administration official also weighed in with sharp criticism. John O. Brennan, the former director of the Central Intelligence Agency, said in a statement that Mr. Trump’s action was “reckless” and would “damage U.S. interests in the Middle East for years to come and will make the region more volatile.”
Administration officials said they expected the blowback from the Palestinians and others, but they asserted that the move would accelerate, not hinder, the peace process.
Mr. Trump promised to move the embassy during the 2016 presidential campaign, a position that appealed to evangelical voters and pro-Israel American Jews, including Sheldon Adelson, the Las Vegas casino magnate. By delivering on that promise, Mr. Trump’s aides said, he was enhancing his credibility as a peacemaker.
The announcement, officials said, was recognition of current and historic reality. West Jerusalem is the seat of Israel’s government, and recognizing it as such would remove ambiguity from the American position, they said.
The process of moving the embassy to Jerusalem will take several years, they noted. As a result, the president plans to sign a national security waiver that allows the administration to keep it in Tel Aviv for an additional six months.
Jerusalem is one of the world’s most fiercely contested swaths of real estate, with each side disputing the other’s claims. Palestinians view East Jerusalem as the capital of a future Palestinian state, and most of the world considers it occupied territory. Jerusalem’s Old City has the third-holiest mosque in Islam and the holiest site in Judaism, making the city’s status a sensitive issue for Muslims and Jews alike.
Mr. Trump, these officials said, would make clear that the United States was not taking a position on whether, or how, Jerusalem would be divided between Israel and the Palestinians. Nor will he take a position on a disputed area of the Old City, known as the Temple Mount to Jews and the Noble Sanctuary to Muslims, which has been a flash point for tensions.
Mr. Kushner and Jason D. Greenblatt, the president’s special envoy, have been working for months to prepare for a negotiation between Israelis and Palestinians. Though they have little to show for the effort publicly, they say they have made progress behind the scenes.
Those efforts, Middle East experts said, will be jeopardized by Wednesday’s announcement. But Mr. Kushner and Mr. Greenblatt, the president’s special envoy, supported the president’s move, officials said.