Despite North Korean leader Kim Jong Un's recent summit with President Donald Trump, North Korea doesn't appear to have abandoned its nuclear ambitions.

Recent reports indicate the country is also working on another ballistic-missile submarine.

Secretary of State Mike Pompeo is scheduled to meet with North Korean officials in Pyongyang on Friday to further discuss denuclearization.

North Korea appears to be working on a new submarine capable of firing nuclear-armed ballistic missiles, according to information gathered by South Korea's military.

Kim Hack-yong, a South Korean lawmaker who until a few weeks ago was head of the legislature's defense committee, told The Wall Street Journal that North Korea appeared to working on the sub at the port of Sinpo on the country's east coast.

An aide to Kim said South Korean intelligence had noticed workers and materials moving at the port, where work on the sub appeared to be taking place at an indoor facility. Kim, whose term as the defense-committee chief recently ended, is a member of the conservative party that has been wary of talks with North Korea.

An underwater test-firing of a submarine ballistic missile shown in an undated photo released by North Korea's Korean Central News Agency on April 24, 2016. KCNA/File Photo via Reuters

US military intelligence noticed similar activity at the port late last year, detecting what appeared to be construction on a new diesel-electric submarine at the Sinpo shipyard, The Diplomat reported in October, citing a US government source.

US intelligence estimates at that time gave the sub a submerged displacement of 2,000 tons and a beam of 36 feet, making it the largest ship built for the North Korean navy.

The Journal report comes as Secretary of State Mike Pompeo visits Pyongyang on Friday, where he is likely to push North Korea for more solid commitments regarding denuclearization. While North Korean leader Kim Jong Un and President Donald Trump pledged at their mid-June summit in Singapore to "work toward" denuclearization, no specific agreement was reached.

The US made some concessions to Pyongyang at that summit, including halting Ulchi Freedom Guardian, a major US-South Korea military exercise scheduled for August. But evidence has emerged suggesting North Korea has not abandoned its nuclear ambitions.

A Hwasong-12 long-range strategic ballistic rocket test-launched on May 15, 2017. KCNA via REUTERS

Five US officials told NBC News that North Korea has increased its production of enriched uranium for nuclear weapons.

While missile and nuclear tests have halted, one official said, "there's no evidence that they are decreasing stockpiles, or that they have stopped their production."

North Korea has one of the world's largest navies by number of ships. South Korea's defense ministry estimates Pyongyang has 430 surface vessels and about 70 submarines.

Many of those subs are thought to be obsolete, but that fleet includes one Gorae-class ballistic-missile sub, which was outfitted with a new missile-launch tube in summer 2017, according to The Diplomat. (South Korea is reportedly looking to buy six US-made P-8 Poseidons, one of the world's most advanced sub-hunting aircraft.)

The sub under construction at Sinpo may be a successor to that Gorae-class boat, advancing a program that US officials consider a threat because it could allow North Korea to achieve greater surprise for a nuclear strike.

"It's too early to say if the North Koreans have defaulted on the Singapore agreement to denuclearize," Yang Uk, chief defense analyst at Seoul-based private think thank the Korea Defense and Security Forum, told The Journal.

"But earlier satellite images have already shown enough evidence proving North Korea has not abandoned its SLBM program," he added, referring to submarine-launched ballistic missiles.