SEOUL • North Korea is expanding an important missile base that would be one of the most likely sites for deploying intercontinental ballistic missiles capable of reaching the United States, two experts on the North's missile programmes said, citing new research based on satellite imagery.

The activities at the Yeongjeo-dong missile base near North Korea's border with China and the expansion of a new suspected missile facility 11km away are the latest indications that North Korea is continuing to improve its missile capabilities, said Dr Jeffrey Lewis and Mr David Schmerler of the Middlebury Institute of International Studies at Monterey in California.

And they come despite President Donald Trump's repeated claims of progress in efforts to denuclearise the North.

Dr Lewis and Mr Schmerler said they were still not sure if Yeongjeo-dong and the new facility under construction in nearby Hoejung-ni, both in the mountainous area near North Korea's central border with China, were separate bases or parts of a larger single operation.

But their geographic locations make them ideal to "house long-range missiles", they said in a report they were preparing.

"The base is located in the interior of North Korea, backed up against the Chinese border," they said. "It is this location that leads us to believe that the general area is a strong candidate for the deployment of future missiles that can strike the United States."

Military planners in Seoul and Washington have long suspected that North Korea would deploy its intercontinental ballistic missiles as close to China as possible to reduce the likelihood of pre-emptive strikes from the US.

"This is one of the important locations in North Korea that our military is monitoring in cooperation with the US," South Korean military spokesman Roh Jae-cheon said on Thursday about the North Korean base. He declined to share further details.

Following his June summit in Singapore with North Korean leader Kim Jong Un, Mr Trump claimed there was "no longer a nuclear threat from North Korea". His administration has also repeatedly claimed progress in talks with North Korea, citing the lack of nuclear and ballistic missile tests since the country launched its Hwasong-15 intercontinental ballistic missile in November last year.

That missile was considered by some experts to be capable of reaching continental US, although North Korea is not yet believed to have the ability to deliver nuclear warheads on such missiles.

In April, Mr Kim placed a moratorium on nuclear or long-range missile tests, saying his country no longer needed to conduct them. He later shut his country's only known nuclear test site and offered to demolish a stand for testing missile engines.

In his meeting with Mr Trump, Mr Kim made a vague pledge to "work towards the complete denuclearisation of the Korean peninsula" in return for "new" relations and security guarantees from Washington. But Mr Kim has yet to clarify whether and when he might dismantle his nuclear warheads and delivery missiles. Nor has he abandoned instructions to "mass produce" these weapons.

In their new report, Dr Lewis and Mr Schmerler said that while closing the test stand would make it harder for the North to design new kinds of missiles, "it would not prevent North Korea from continuing to mass produce and deploy existing types of nuclear-armed missiles that can strike the US". Those missiles are being deployed at bases throughout North Korea despite the Singapore meeting, they said.

"Any denuclearisation agreement would require North Korea to allow international inspectors to determine that these units are no longer armed with nuclear weapons," they said.

NYTIMES