Earlier this year, President Trump created the Board of Peace, an international organization that was initially aimed at resolving the long-running war in Gaza.

On Thursday, Mr. Trump said on social media that the organization had reached an agreement for Hamas to begin disarming in the Gaza Strip. Details of the plan are not known, and disarmament would be a monumental undertaking. Hamas, which specializes in insurgent tactics, has long regarded giving up all its weapons as tantamount to surrender.

The Board of Peace was central to Mr. Trump’s plan for peace in Gaza. After brokering a cease-fire between Israel and Hamas last year, the board was established, backed by a U.N. mandate, to oversee the reconstruction effort in Gaza.

Much of that effort hinged on the disarmament of Hamas. Board officials drafted a plan for the demilitarization of Gaza, and Mr. Trump chose a U.S. general to lead the military force that would be deployed to Gaza to disarm Hamas and occupy the Palestinian enclave during the yearslong campaign to rebuild it.

Under a previous draft plan, Hamas would be required to surrender its weapons capable of striking Israel, while allowing the group to retain some small arms, at least initially. Officials at the time cautioned that details of the plan could change.

Mr. Trump has moved to broadly expand the scope of the Board of Peace to include other diplomatic efforts led by the Trump administration in what appeared to be an effort to build an alternative to the United Nations that places him in charge. Under the organization’s charter, Mr. Trump is the board’s chairman, and could play a central role in the board even after leaving the presidency. Mr. Trump can hold the chairmanship “until he resigns it,” a U.S. official said.

Countries must contribute more than $1 billion to become permanent board members, but can join without cost for at least the first three years.

The more than 20 founding members of the organization, in addition to the United States, include Argentina, Hungary, Indonesia, Pakistan, Saudi Arabia, Turkey, the United Arab Emirates and Qatar.

Several longtime European allies of the United States, such as France and Spain, have said that they would not join the board at this time, saying that its charter raised serious questions about international law and respect for the U.N.