SEOUL, South Korea -- North Korea has significantly expanded a key long-range missile base near the Chinese border, according to reports citing new satellite images that provide more evidence the communist state is persisting with its nuclear weapons program despite U.S.-led diplomatic efforts to stop it.
The findings come as nuclear talks between Pyongyang and Washington have stalled months after leader Kim Jong Un met with President Donald Trump in Singapore and promised to work toward the "complete denuclearization of the Korean Peninsula."
The commercial imagery, which was analyzed by researchers at the California-based Middlebury Institute of International Studies at Monterey, shows construction taking place at the existing missile base called Yeongjeo-dong and a second site known as Hoejung-ri, about 7 miles away, CNN and NPR reported Wednesday.
The location of the base in the remote, mountainous area of Yeongjeo-dong has long been known to U.S. intelligence agencies, but the apparent new facility had not been previously publicly identified, according to the report, which added that it's unclear whether the new base is separate or an extension of the existing facility.
"This missile base could be a future deployment site for missiles that could strike the United States," Catherine Dill, a senior research associate with the institute, told CNN.
The bases are believed to be shelters for mobile missile launchers, not production facilities, according to NPR.
South Korea's military declined to comment on the report but said during a press conference Thursday that Yeongjeo-dong is one of the major areas that is under surveillance by Seoul and Washington.
There was no immediate response to the report from the U.S. administration, which has dismissed similar findings in the past.
North Korea also announced it had tested an unspecified "newly developed ultramodern weapon" last month.
Trump, meanwhile, has said he hopes to hold a second summit with Kim early next year.
National Security Adviser John Bolton said the president thinks another meeting would help move the process forward.
"They have not lived up to the commitments so far. That's why I think the president thinks another summit is likely to be productive," Bolton said Tuesday during a forum.
"If the North Koreans follow through on the commitments they made in Singapore President Trump will deserve the Nobel Peace Prize. He opened the door for them, now they have to walk through it," Bolton added.
Experts are quick to point out the North Koreans didn't make any concrete commitments during the Singapore summit and subsequent negotiations on details have failed to progress amid disagreements and conflicting demands.
The communist state has suspended its nuclear- and missile-testing program, which raised fears of a nuclear war last year. It also destroyed its main nuclear-testing site and begun dismantling a missile launch site.
But it has called for a reciprocal approach that would include rewards for steps taken such as easing international sanctions and declaring a formal end to the stalemated 1950-53 Korean War.
Trump and his allies have insisted that they will maintain economic pressure on the North until what it calls final, fully verified denuclearization is achieved.
On the peninsula, the Koreas pressed forward to improve relations with the implementation of a far-reaching military agreement reached in September.
South Korea's defense ministry said the two sides will exchange inspection teams across their heavily fortified border next week to verify promised work to remove several front-line guard posts.