BEIRUT, Lebanon — In unveiling his plan Tuesday for solving the conflict between Israel and the Palestinians, President Trump confidently declared that Arab countries would play a key role in its success.

But none of the United States’ Arab allies formally endorsed the plan or made concrete commitments to back it, raising questions about how helpful they will really be in bringing it to fruition.

Mr. Trump announced the plan in an appearance at the White House with Prime Minister Benjamin Netanyahu of Israel, describing it both as necessary for Israel’s security and as an opportunity for the Palestinians to govern themselves and grow their economy.

There are “many, many countries who want to partake in this,” he told Mr. Netanyahu, predicting that “you are going to have tremendous support from your neighbors and beyond your neighbors.”