North Korea says it has suspended nuclear and long-range missile tests and plans to close its nuclear test site. (AP)

During the same week that CIA Director Mike Pompeo flew to Pyongyang for secret talks on ending North Korea’s nuclear program, U.S. weapons experts were studying a construction site near the Chinese border for clues that North Korean officials might be moving in the opposite direction.

Analysts pored over satellite images of the northern city of Chongsu, keying in on a large, red-roofed building that some experts suspect is a factory for making an ultrapure form of graphite. The material is essential for making nuclear reactors of the kind North Korea uses domestically, and, according to a new report, Pyongyang has recently attempted to sell the same nuclear-grade graphite to customers overseas.

The CIA has declined to comment about the mysterious building, the true purpose of which remains uncertain. But the questions over the construction project underscore a key difficulty in evaluating North Korea’s proposals to freeze or give up portions of its nuclear program: North Korea has a long history of concealing illicit weapons activity from foreign eyes.

The Trump administration on Friday welcomed North Korea’s surprise announcement that it would halt further testing of nuclear weapons and long-range missiles and forswear transferring weapons technology to other countries. But former U.S. officials and weapons experts urged caution, noting the difficulties of verifying compliance and Pyongyang’s well-earned reputation for cheating.

“The Kim family regime’s past behavior gives new meaning to the traditional arms control challenge of ‘trust but verify,’ ” said Robert Litwak, director of international security studies at the Woodrow Wilson International Center for Scholars, a Washington think tank.

North Korean leader Kim Jong Un announced the suspension of testing during a meeting of the ruling Workers’ Party of Korea’s central committee. The dictator made no mention of giving up the country’s nuclear stockpile, variously estimated to contain between 20 and 100 warheads. He said a freeze was possible because North Korea had “verified the completion of nuclear weapons” and was now a full-fledged nuclear weapons state.

The announcement, which came weeks before an expected U.S.-North Korean summit, included a pledge by Kim that he would “never use nuclear weapons unless there is a nuclear threat” or “transfer nuclear weapons or nuclear technology” to outsiders.

The promises fall well short of the Trump administration’s stated goal of a full “denuclearization” of the Korean Peninsula. Some analysts worry that Kim is seeking to outmaneuver his opponents ahead of the negotiations, offering concessions so he can pocket the dramatic gains in nuclear and missile technology achieved over the past two years.

Moreover, it is unclear how the United States and its allies would reliably verify a suspension of key facets of North Korea’s nuclear program or confirm that it has stopped selling weapons components to partners overseas. While any new test of a nuclear weapon or long-range missile would be spotted immediately, North Korea could conceivably continue assembling nuclear warheads in secret while adding to its growing stockpile of weapons-grade uranium and plutonium, weapons experts say.

Unlike Iran, which is subject to intensive inspections under the nuclear agreement signed in 2015, North Korea does not allow international inspectors to visit its nuclear facilities. And having endured decades of U.N. trade restrictions, Pyongyang possesses an unrivaled record for concealing its weapons production facilities and secretly selling components to foreign customers.

In the 1990s, after North Korea struck a deal with the Clinton administration to stop making plutonium, Pyongyang switched to a covert uranium production program that went undetected for years. U.S. intelligence agencies suspect North Korea was the principal architect of a secret Syrian nuclear reactor destroyed by Israeli warplanes in 2007 and a supplier of missile technology to several countries, including Iran and Burma.

“No country has been more consistent in its willingness to sell its weapon systems to other bad actors,” Victor Cha, a former director of Asian affairs with the George W. Bush administration’s National Security Council, told the House Foreign Relations Committee at a hearing earlier this month. “The U.S. must consider seriously that Pyongyang would do the same with its nuclear and ballistic missile capabilities.”

According to weapons experts, the suspected graphite production facility in Chongsu could help North Korea achieve multiple goals, allowing its weapons program to quietly advance while creating an additional source of badly needed export revenue.

The site was identified in a report released late Friday by the Institute for Science and International Security, a Washington nonprofit that researches nuclear weapons programs. David Albright, the institute’s founder and a former U.N. weapons inspector, said the facility came to light partly because of a glossy marketing brochure North Korea has been providing to select customers offering to sell nuclear-grade graphite. The black mineral, when purified through a manufacturing process, is used in making nuclear reactors and missile components, in addition to other industrial applications.

With guidance from an unidentified “knowledgeable official,” the institute’s experts were able to spot the suspected facility inside a guarded industrial zone near the banks of the Yalu River. A sequence of satellite images taken over seven years shows its construction on the grounds of a defunct coal-burning power plant, which was dismantled and replaced by modern industrial buildings “surrounded by a new security perimeter wall with a security checkpoint building” at the front entrance, the institute’s report states.

While the report offers no concrete proof that the facility is intended for making nuclear-grade graphite, Albright cites multiple strands of evidence pointing to North Korea’s interest in expanding its production of the material. Pyongyang already possesses one graphite factory, and in recent years, North Korea’s atomic energy agencies have acquired new equipment and dispatched scientists to China for advanced training in producing the substance.

“This example illustrates why the United States needs to obtain a well-defined, verified commitment from North Korea not to proliferate nuclear weapons, fissile materials and nuclear and nuclear-related goods,” Albright said.

Regardless of the building’s purpose, independent weapons experts say, it is crucial that North Korea allows access to such facilities. Without such transparency, it would be difficult to take Kim’s pledges seriously, analysts say.

“There is good reason to be skeptical, or at least realistic,” Jon Wolfsthal, a former senior director for arms control in the Obama administration’s National Security Council, said in a Twitter post hours after the freeze was announced. “It is, after all, North Korea.”