SEOUL, South Korea — North Korea demonstrated its ballistic missile capabilities by launching two midrange missiles on Wednesday, after the leaders of the United States, Japan and South Korea gathered in the Netherlands to discuss the North’s nuclear threats.

In North Korea’s first tests of midrange projectiles in nearly five years, two Rodong missiles blasted off from mobile launching vehicles from Sukchon, north of Pyongyang, early Wednesday and flew 403 miles before landing in the sea between North Korea and Japan, said Kim Min-seok, a spokesman of the South Korean Defense Ministry.

“By launching them from mobile vehicles which are difficult to monitor and allow North Korea to fire missiles from anywhere it wanted, the country appeared to show off its ability to attempt a surprise attack,” Mr. Kim said. “This is a serious provocation against South Korea and the international community.”

Mr. Kim would not say how it was known that mobile vehicles had been involved, but North Korea has displayed an array of mobile launching pads known as transporter erector launchers in recent military parades.