SEOUL—North Korean leader Kim Jong Un examined a new submarine that experts believe could carry multiple missiles, including those with nuclear capabilities, in Pyongyang’s latest display of military strength as denuclearization talks with Washington remain gridlocked. North Korean state media, in a Tuesday report, didn’t provide details about the submarine’s tactical abilities or its physical size. But the North plans to deploy the new submarine soon, state media said. Based on photo analysis, military experts say the vessel represents an advancement in the North’s maritime weaponry because it increases the number of submarines capable of launching missiles and may be sizable enough to carry multiple projectiles. The Kim regime’s submarines were previously believed to be capable of loading only a single missile. Mr. Kim inspected the black-and-gray submarine at an arms factory on the country’s eastern coast, an area believed to house most of the North’s submarine bases. The North Korean leader hailed the new vessel as “another demonstration of the might of our defense industry,” according to state media, which didn’t specify when Mr. Kim conducted the inspection. South Korea’s Defense Ministry said it would closely monitor the situation but didn’t comment further. The State Department didn’t immediately respond to a request for comment. The submarine display comes as disarmament talks between the U.S. and North Korea remain stalled. The deadlock has irked Pyongyang, which sees a nuclear deal as a possible way to deliver economic relief. Mr. Kim’s inspection also coincides with a visit to Seoul by national security adviser John Bolton, who is meeting with South Korean officials. President Trump said Monday that the U.S. and North Korea have yet to schedule a date to reconvene working-level nuclear talks, though he added that the two sides have traded some correspondence. “When they’re ready, we’ll be ready,” Mr. Trump said. The submarine inspection follows three weapons tests and an air drill by North Korea in recent months. Security experts say the moves are meant to serve as reminders to the U.S. and South Korea of what could happen if diplomacy falters. The Kim regime has reacted angrily to a planned U.S.-South Korea military exercise and Seoul’s purchases of American stealth jets. North Korea last week suggested that going ahead with the exercise could prompt it to restart long-range missile tests, but experts say such warnings through state media are a familiar tactic by Pyongyang. North Korea warned in mid-July that the country might resume long-range missile testing in response to the U.S.’s scheduled military drills with South Korea. WSJ’s Gerald F. Seib explains this stage of U.S.-North Korean diplomacy. Photo: Reuters North Korea has tested submarine-launched ballistic missiles with mixed results over the past five years. In 2016, it successfully tested a submarine-launched ballistic missile, called Pukguksong-1, though its reach appeared to be limited in distance and scope, experts said. Pyongyang’s new submarine appears larger than the country’s past models. The underwater technology also is tougher for the U.S. and South Korean militaries to detect, posing a big threat to Tokyo and Seoul, said Prof. Kim Young-joon of the Korea National Defense University, which is run by the South Korean Defense Ministry. Publicizing the submarine technology on the same day as Mr. Bolton’s Seoul visit likely wasn’t accidental, said Kim Dae-young, a researcher at the Korea Research Institute of National Strategy, a Seoul-based think tank. Mr. Bolton is a frequent target of North Korean state media, which has called him a warmonger and human scum. “The submarine is a welcome gift to Bolton,” Mr. Kim, the researcher, said. “This is the North Korean way of throwing a fit when it is frustrated.” North Korea watchers had suspected the isolated regime could be enhancing submarine-missile technology. Using satellite imagery, the Stimson Center, a Washington-based think tank, published a report last month that spotted construction activity, including the movement of equipment and parts to submarines, near a suspected North Korean naval base. Pyongyang has about 70 submarines, according to South Korean estimates, though military experts believe only one is advanced enough to fire missiles. The Kim regime represents the “most immediate threat” to Washington, and there is no doubt the country is continuing to develop nuclear weapons and long-range ballistic missiles, said U.S. Navy Adm. Philip Davidson, at a weekend security conference.