WASHINGTON — Jason Greenblatt is leaving as President Donald Trump’s envoy for brokering an Israeli-Palestinian peace deal, senior officials told NBC News on Thursday.

His departure, confirmed by the president on Twitter, is the latest sign that optimism within the Trump administration for the long-awaited plan’s prospects may be fading. Release of the plan, under development since the start of administration, has already been delayed several times, most recently until sometime after the Israeli elections on Sept. 17.

Avi Berkowitz, a longtime aide to Trump senior adviser and son-in-law Jared Kushner, will replace Greenblatt in his role as special adviser for international negotiations, the administration officials said.

And State Department senior adviser Brian Hook, who has spearheaded the administration’s Iran policy efforts, is expected to take on a larger role on the peace plan and work more closely with Kushner as the administration’s Iran policy increasingly merges with Kushner’s peace initiative.

Trump confirmed Greenblatt’s departure midday Thursday, saying that his former attorney would be leaving to “pursue work in the private sector.” Greenblatt is expected to remain with the administration for the coming weeks if not months, officials said.

....His dedication to Israel and to seeking peace between Israel and the Palestinians won’t be forgotten. He will be missed. Thank you Jason! — Donald J. Trump (@realDonaldTrump) September 5, 2019

The substance of the plan that the White House has been championing for months remains opaque, though it has two parts: one to address economic issues, and the other dedicated to political divides.

The economic portion of the plan was released in June to little fanfare during a U.S.-organized summit in Bahrain that neither Israeli nor Palestinian officials attended. It called for an investment of about $50 billion to lift up Palestinians economically.

Yet since then, there has been little sign of progress toward rolling out the broader plan that will address the thorniest issues, such as whether the Palestinians will get an independent state, the status of Palestinian refugees, the city of Jerusalem, and Jewish settlements in the West Bank.

Palestinian officials, incensed by the administration’s policies toward Palestinians, have already rejected the plan. Palestinian leaders and the White House have not formally communicated since 2017, when Trump infuriated the Palestinians by recognizing Jerusalem as Israel’s capital and moving the U.S. Embassy there in a reversal of longstanding U.S. policy that Jerusalem’s future should be determined by negotiations. The Palestinians claim part of Jerusalem for the capital of a future independent state.

Israeli Prime Minister Benjamin Netanyahu’s political troubles at home have also created significant complications for the plan. Now Israel’s longest-serving prime minister, Netanyahu failed to put together a governing coalition after Israel’s last elections in April, leading to the new elections this month.

“I would like to thank Jason Greenblatt for his dedicated work for security and peace, and for never hesitating for a moment to speak the truth about the State of Israel in front of all its abusers,” Netanyahu said in a statement in Hebrew.

The shift coincides with rising U.S.-Iran tensions and Trump saying he’s willing to meet with Iranian President Hassan Rouhani within weeks. Iran also is expect to be a large focus of the gathering of world leaders, including Trump, in New York this month for the United Nations General Assembly.

Greenblatt, a real estate lawyer who worked for Trump's business for roughly two decades, did not have any formal diplomatic experience before being tapped for the Mideast peace envoy role at the start of the Trump administration.