Since the breakdown of the talks in Hanoi, Vietnam’s capital, many analysts, including Mr. Moon’s conservative critics at home, have voiced concern over the future of the U.S.-South Korean alliance. Mr. Moon has alarmed conservatives by declaring that he would push for joint North-South Korean economic projects, even though the United States insists that this is not the time to ease economic pressure on the North.

A day after the Hanoi summit collapsed, Mr. Moon said he would discuss with Washington the possibility of reopening of a joint inter-Korean factory park in the North Korean city of Kaesong, as well as resuming South Korean tours of Diamond Mountain, a resort area in the North. He later told his government to look for ways to help advance the dialogue between Washington and the North “through improving South-North Korean relations.”

But Washington fears such projects will weaken the enforcement of United Nations sanctions against the North. In Hanoi, Mr. Trump “made clear the United States expects complete denuclearization before sanctions relief,” said Robert Palladino, deputy spokesman for the State Department in Washington.

Mr. Trump’s decision to halt the Hanoi summit without a deal shocked Mr. Moon as much as Mr. Kim, who had taken a 65-hour train ride to Vietnam expecting sanctions relief.

Mr. Moon is now desperate to help restart dialogue between Pyongyang and Washington to prevent them from returning to missile and nuclear tests and threats of war, which would destroy the rapprochement Mr. Moon has helped build on the peninsula in the past year.