“It’s disappointing,” Trump said during a brief photocall with Czech Republic Prime Minister Andrej Babis.
“We’ll see. We’ll let you know in about a year,” he added, without elaborating.
South Korea’s JoongAng Ilbo and Donga Ilbo newspapers first reported the activity at the North Korean factory, citing lawmakers briefed by the National Intelligence Service.
White House national security adviser John Bolton said on Thursday that Trump was still open to additional talks with North Korea over denuclearization.
Bolton, a hard-liner who has argued for a tough approach to North Korea in the past, said it was too soon to make a determination on the reports of the North Korean activities.
The February 27-28 Vietnam summit collapsed over differences on how far North Korea was willing to limit its nuclear program and the degree of U.S. willingness to ease economic sanctions.
(Production: George Tamerlani)