United States President Donald Trump on Tuesday said there was a “substantial chance” that his historic summit with North Korea’s leader Kim Jong-un next month in Singapore may be delayed, reported the BBC. Trump announced it as he received South Korean President Moon Jae-in at the White House to discuss the summit.

“We’ll see what happens,” Trump said, according to The New York Post. “If it doesn’t happen, maybe it will happen later. You never know about deals. It may not work out for June 12.”

Last week, North Korea threatened to cancel the summit between Kim Jong-un and Trump after the United States conducted a joint military exercise with South Korea. A day later, South Korea said it intended to mediate the talks.

Tensions between the US and North Korea escalated in 2017 after Pyongyang stepped up efforts to boost its nuclear weapons programme. The summit in Singapore was aimed at the denuclearisation of the Korean peninsula.