"This renewed activity, taken just two days after the inconclusive Hanoi summit between President Donald Trump and Chairman Kim Jong-un, may indicate North Korean plans to demonstrate resolve in the face of USA rejection of North Korea's demands at the summit to lift five UN Security Council sanctions enacted in 2016-2017", CSIS speculated.
The United States says it wants a full "denuclearisation" of North Korea, but the Trump-Kim summit's abrupt ending without agreement left the future of their talks uncertain.
38 North's analysis of the Sohae construction noted that efforts to rebuild the missile launch pad and engine test stand began sometime between February 2 and March 2, so the work might have started before the Trump-Kim summit in Hanoi.
South Korea's National Intelligence Service provided the assessment about the North's Tongchang-ri launch site to lawmakers during a private briefing Tuesday. "We also continued to observe indications of the ongoing use of the reported centrifuge enrichment facility", Amano said. North Korea didn't immediately respond in its state media.
The Center for Strategic and International Studies think tank released a separate report, also citing satellite imagery, that concluded North Korea was "pursuing a rapid rebuilding" at the site.
Speaking at a news conference after meeting with Abe, Sakie Yokota, the 83-year-old mother of Megumi Yokota, who was abducted at age 13 in 1977, said she is not sure when Abe will meet with Kim but she is hoping an "exchange of words between the Japanese and North Korean leaders" will take place.
At the engine test stand, the website said it appears that the engine support structure is being reassembled.
"Analysts cautioned that the site has never been used to launch an intercontinental ballistic missile and there is no evidence to suggest a test is imminent, but the site has been used to test missile engines and past satellite launches have helped scuttle talks with the United States", Channel News Asia added.
Trump and Kim met last week in Vietnam to negotiate on getting rid of North Korea's nuclear arsenal, but the summit broke up early with no progress. After several failures, North Korea successfully launched a satellite from the site in 2012.
Director General Yukiya Amano told the agency's board members on Monday that the IAEA has not observed any indications of the operation of the five-megawatt reactor at the Yongbyon nuclear complex since early December of past year.
"We'll look at ramping those sanctions up, in fact", Bolton warned.
On Tuesday, a top USA official said the United States would look at increasing sanctions against North Korea if Kim did not end his country's nuclear weapons program. "The U.S. government believes there are at least two additional enrichment (facilities)", he said.
A statement issued by Kim and Moon categorized the steps as parts of a broader goal to make the Korean Peninsula free from nuclear weapons and nuclear threat. Experts say, however, that nuclear missiles and satellite rockets use similar technology.