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The U.S. is "watching in real time" developments at a long-range rocket site that North Korea is believed to have rebuilt, a senior State Department official said Thursday.

The Sohae facility has been used in the past for satellite launches that employ similar technology to what is required to fire intercontinental ballistic missiles. It is now back to "normal operating status," according to new commercial images and analysis from two teams of researchers.

According to the official, North Korean leader Kim Jong Un had pledged to dismantle Sohae both at his June summit with President Donald Trump in Singapore, and again during a meeting with South Korea's President Moon Jae-in in September. Kim also said independent inspectors would be allowed to verify compliance on the ground, he said.

"The intent of the North Koreans in this matter is known only to them, we don't know why they are taking these steps,” the State Department official told reporters. “They need to keep their commitments to the president of the United States."

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The official added the Trump administration would be looking for clarification from Pyongyang.

The State Department official said that while it is important for North Korea to act as it had promised, the U.S. side did not see Sohae as a "critical" part of Pyongyang's missile program because such tests have most recently been from mobile launchers.

"I don't want to diminish the concerns we have, but I also don't want to exaggerate their importance to their infrastructure,” the official said.

Meanwhile, The Associated Press reported that South Korea's military said it was carefully monitoring North Korean nuclear and missile facilities. NBC News could not immediately independently verify the report.

President Donald Trump met North Korean leader Kim Jong Un in Hanoi, Vietnam, last monthl Evan Vucci / AP

On Feb. 28., high-stakes talks between Trump and Kim collapsed, with the president saying that the North had demanded a full removal of sanctions in exchange for shuttering the Yongbyon nuclear facility.

North Korea later disputed this account, saying Pyongyang had demanded only partial sanctions relief in exchange for shuttering the nuclear complex. A State Department official subsequently confirmed that was the case.

Images taken on March 6 show activity that "speaks to the ease with which [North Korea] can reverse steps it might take toward denuclearization in the future," according to an analysis by Joseph Bermudez and Victor Cha of Beyond Parallel, a research project funded by the think tank Center for Strategic and International Studies.

The analysts cautioned that the imagery has yet to show a missile being moved to the launch pad.

Researchers from 38 North arrived at the same conclusion that the Sohae facility "appears to have returned to normal operational status."

Trump has said he would be “very very disappointed” if North Korea was rebuilding the long-range rocket site.

The president has repeatedly used the fact that North Korea has not launched a missile since late 2017 as a sign that the diplomatic process is working.