WASHINGTON — During the same week that CIA Director Mike Pompeo flew to Pyongyang for secret talks on ending North Korea’s nuclear program, US 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 poured 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 new 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 as well as 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 US-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, trying to secure a lessening of economic sanctions and a chance to sit down with President Trump on an essentially equal basis — as head of a nuclear-armed nation.
They suspect he is offering the concessions as a way to preserve in the long run the dramatic gains in nuclear and missile technology achieved over the past two years.
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 new nuclear warheads in secret while also adding to its growing stockpile of weapons-grade uranium and plutonium, weapons analysts say.
Unlike Iran, which is subject to intensive inspections under the nuclear agreement signed in 2015, North Korea does not currently allow international inspectors to visit its nuclear facilities. And having endured decades of UN 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.
US intelligence agencies believe 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 Myanmar.
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 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.
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 and security facilities, the report said.