Researchers at the Middlebury Institute of International Studies, who analyzed the satellite imagery, said this is the first time the Hoejung-ri site has been publicly identified, describing the work being done there as "significant".
But National Security Adviser John Bolton said Tuesday that there should be another summit with Kim.
"Beijing may be using this opportunity to share the information [on the summit results] with North Korea", Zhang said.
North Korea's foreign minister was due to arrive in key ally China Thursday for talks that come amid stalled efforts to persuade Pyongyang to dismantle its nuclear weapons programs.
US officials did not immediately comment on the open-source findings by researchers.
"We watch North Korea very closely".
The US has demanded that Pyongyang take substantial steps towards denuclearization while the latter has asked for security guarantees before it signs any peace agreement. "We will not discuss matters of intelligence", Pentagon spokesperson Lt. Col. Chris Logan told CNN.
However, Ri's visit follows hard on the heels of last week's US-China summit in Argentina, which included discussion of North Korea.
"Moreover, in the past year North Korea has significantly expanded a nearby facility that appears to be another missile base".
A spokesman for the ministry refused to comment further, saying "it is not appropriate for our military to officially acknowledge the content of a foreign media report".
Although the construction does not technically violate any agreement between the USA and South Korea, it could be seen as evidence that numerous sites remain active.
"That's why I think the president thinks that another summit is likely to be productive", Bolton said during the Wall Street Journal's CEO conference.
Trump on Sunday had said he planned to hold a second summit with Kim that could take place in January or February and three places were being considered for the meeting.
The North Korea official "achieved a self-reliant military line and actively contributed to the modernization of North Korea's defense industry", Choe said.
Requests to visit that site and other missile sites were denied by leader Kim Jong Un, violating an agreement to end North Korea's missile program. "North Korea has a big job to do".
Some analysts say the activity is geared toward the continual development of nuclear-capable intercontinental ballistic missiles that could reach the US mainland.
"North Korea may wish to accelerate restoring its relations with China and its economic and political support to offset the risk on the possible failure of the talks", Jin said.
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