President Donald Trump said Tuesday that there’s a substantial chance that his summit with North Korean leader Kim Jong Un “may not work out” for June.

Trump made the remark while he met with Moon Jae-in, South Korea’s president, for pivotal discussions ahead of the American president’s planned meeting with the North Korean dictator.

The summit is scheduled for June 12 in Singapore, which is widely viewed as a neutral site. Yet doubts continue to grow about whether the meeting will actually take place. Trump’s remarks Tuesday were the strongest indication yet that the summit might not happen as planned.

Last week, North Korea said it would reconsider whether to hold the meeting after abruptly canceling talks with South Korea amid joint military drills with the U.S. on the Korean Peninsula.

The South Korean president struck a more optimistic note, however. Moon pushed back against U.S. skepticism about the meeting and said he is confident that Trump will make the summit a success.

In his remarks Tuesday, Trump did say that he believes Kim is serious about denuclearization. The Trump administration has made it clear that it wants North Korea to give up its nuclear-weapons program.

The president added that the meeting won’t happen if certain conditions aren’t met. He also said a meeting could happen further down the road if it doesn’t occur next month as planned.

No sitting U.S. president has met face-to-face with a North Korean leader.

This story is developing. Please check back for updates.