South Korean President Moon Jae-in said on Thursday that he is “baffled and very regretful that the North Korea-U.S. summit planned for June 12 is not happening.”

According to ABC News, the South Korean government has convened a national security meeting over President Donald Trump’s sudden cancellation of the summit on Thursday morning.

South Korean officials said they did not know in advance that Trump was sending a cancellation letter to dictator Kim Jong-un. The officials said they are “trying to find out what President Trump meant and what the letter exactly means.”

President Moon invested much effort and political capital in the diplomatic opening to North Korea, culminating in his own summit with Kim Jong-un in April. The first harbinger of serious trouble for the Trump-Kim meeting came last week when North Korea launched vitriolic criticism of joint U.S.-South Korean military drills and canceled a planned meeting with South Korean officials.

At a White House meeting with President Trump on Wednesday, Moon anticipated inter-Korean dialogue would resume after the Max Thunder joint air combat exercises conclude on May 25.