SEOUL — North Korea sought to increase pressure on the United States ahead of proposed nuclear talks, declaring Tuesday that it has built a new submarine and instructed its commanders to upgrade their military readiness.

The announcement in state media demonstrated the North’s continued expansion of its military capabilities and its frustration about a lack of progress in nuclear diplomacy with Washington, defense analysts said.

Security experts and South Korean officials have long suspected that North Korea, which already has a sizable submarine fleet, has been working to develop a new submarine that could launch nuclear-armed ballistic missiles.

Kim Jong Un, the country’s leader, inspected the new vessel, which was built under his “special attention” and will soon operate along the country’s east coast, North Korea’s state media said. It did not mention when or where the inspection took place but said Kim “expressed great satisfaction” about the submarine’s design, intended to be capable of implementing Pyongyang’s strategic goals.

Those comments indicated that the submarine “appeared designed to carry nuclear-capable ballistic missiles for strategic deterrence,” said Ankit Panda, an adjunct senior fellow at the Federation of American Scientists.

Pyongyang hinted last week that it might resume nuclear and long-range missile tests if the United States and South Korea go ahead with planned joint military exercises in August. It accused President Trump of reneging on a commitment to suspend the exercises.

“While pursuing nuclear talks on the one hand, Kim is demonstrating that North Korea will continue to build up its military capabilities,” said Lee Ho-ryung, a researcher at the Korea Institute for Defense Analyses in Seoul. “North Korea has been developing a submarine over the past few years, but the timing of the announcement appears to be a diplomatic decision, sending a message to Washington.”

Trump and Kim agreed at a June 30 meeting to resume working-level talks on the nuclear issue. The president said Monday that there has been “very positive correspondence” with North Korea recently, but that no date has been set for a meeting.

A summit between the two leaders in Hanoi in February did not produce an agreement on denuclearization. North Korea accused the United States of demanding too much while not offering offer sufficient sanctions relief in return for disarmament steps.

Tuesday’s developments came as national security adviser John Bolton traveled to Seoul for meetings with South Korean officials.