WASHINGTON — President Donald Trump suggested Tuesday that a planned historic meeting with North Korea’s Kim Jong Un could be delayed. He said, “There’s a very substantial chance that it won’t work out” for June 12.

Trump raised the possibility that the meeting could be pushed back during a White House meeting with South Korea President Moon Jae-in, trying to coordinate strategy as concerns mounted over ensuring a successful outcome for the North Korea summit.

Trump told reporters: “If it doesn’t happen, maybe it happens later,” reflecting recent setbacks in efforts to bring about reconciliation between the two Koreas.

The North pulled out of planned peace talks with the South last week, objecting to long-scheduled joint military exercises between U.S. and Republic of Korea forces. And the North threatened to abandon the planned Trump-Kim meeting over U.S. insistence on denuclearizing the peninsula, issuing a harshly worded missive that the White House dismissed as a negotiating ploy.

“There are certain conditions that we want,” Trump said Tuesday. He added if they aren’t met, “we won’t have the meeting.” He declined to elaborate on those conditions.

Trump said “there’s a very substantial chance” that the meeting won’t take place on June 12. “That doesn’t mean it won’t work out over a period of time,” he said. “But it may not work out for June 12. But there is a good chance that we’ll have the meeting.”

Moon said in the Oval Office that the “fate and the future” of the Korean Peninsula hinged on the talks, telling the U.S. president that they were “one step closer” to the dream of a denuclearized Korean Peninsula.

Trump said he’d noticed “a little change” in Kim Jong Un’s “attitude” after Kim took a second trip to China this month in the run-up to the summit. “I don’t like that,” Trump said.

Trump said he hoped that Chinese President Xi Jinping was committed to the goal of denuclearizing the Korean Peninsula, calling him a world-class poker player. But he said he was displeased by China’s softening of border enforcement measures against North Korea.

Trump encouraged Kim to seize the opportunity for the meeting and to make a deal to abandon his nuclear program, pledging not only to guarantee Kim’s personal security, but also predicting an economic revitalization for the North.

“I will guarantee his safety, yes,” Trump said, if Kim agrees to complete, verifiable, and irreversible denuclearization. He said if an agreement is reached, China, Japan and South Korea would invest large sums to “make North Korea great.”

Trump said the long-term status of the peninsula will be up to the North and South, and that the immediate goal for his summit is “two successful Koreas.”

He added, “Ultimately, maybe someday in the future” you’ll “go back to one Korea.”

The two Koreas both seek reunification of the divided Korean Peninsula on their own terms but it has always been a distant aspiration because of the incompatibility of their political systems and their mutual suspicion. Any move toward reunification would first likely require a peace settlement to formally end the 1950-53 Korean War, which the two Koreas say they are aiming for.

Fresh questions were raised Tuesday about North Korea’s goals and motives, with a Pentagon report to Congress that says nuclear weapons are central to North Korea’s strategic goal of ensuring the perpetual the rule of the Kim family dynasty. The report on North Korea’s military capabilities was based on an assessment of developments in 2017 and was provided to Congress in April. It was posted online by an anti-secrecy group.

AP writer Ken Thomas contributed.