South Korea’s National Intelligence Service said on Tuesday that North Korea is rebuilding the Sohae missile launch facility, a site it pledged to dismantle last year during the early stages of negotiations with the United States.

The South Korean report was backed up by analysis from two American organizations, the Center for Strategic and International Studies (CSIS) and 38 North.

CSIS looked at satellite images of the facility, also known as Tongchang-ri, taken on March 2 and concluded North Korea is “pursuing a rapid rebuilding of the long-range rocket site at Sohae.”

The construction and maintenance in progress at the site is the first significant activity since August 2018, suggesting a “deliberate and purposeful” effort to bring it back online.

“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 U.S. rejection of North Korea’s demands at the summit to lift five UN Security Council sanctions enacted in 2016-2017,” CSIS speculated.

The “Beyond Parallel” project at CSIS previously issued a string of reports describing missile launch facilities well-known to South Korean and U.S. intelligence but not formally declared by North Korea. The Beyond Parallel project is headed up by Victor Cha, who was once a candidate to become U.S. ambassador to South Korea but was withdrawn from consideration after disagreeing with the Trump administration’s North Korea policy.

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. In fact, 38 North mentioned construction materials have been delivered to the site over the past several months and stacked near the structures that are now being repaired.

“Given how much has been done at this site, it looks like more than a couple days’ worth of activity. It’s hard to say if it happened immediately after the summit and they just rushed everything – I guess it’s possible – but it’s more likely that it started just before,” 38 North Managing Editor Jenny Town told the Washington Post on Wednesday.

Jeffrey Lewis of the Middlebury Institute of International Studies told the Post there was ambiguous activity at Sohae as early as February 18, suggesting North Korea “may have known the summit was not going to go well,” so preparations were made for a new round of provocations.

Lewis thought a satellite launch from Sohae could be the perfect provocation since North Korea’s allies in Russia and China would probably declare it legal, the United States would be more likely to object, and South Korea would have to choose between indulging Pyongyang and risking its own diplomatic progress with its belligerent neighbor.

38 North pointed out that Pyongyang’s promise to dismantle Sohae was followed by an initial burst of rapid progress, followed by little detectable change since August 2018.

The rail-mounted transfer building now boasts restored walls and a repaired roof that appears slightly higher than the previous enclosure, while the support structure at the engine test stand has been reassembled, with new roofs for the fuel and oxidizer buildings. Cranes and other vehicles can be seen around both the transfer building and the engine test stand.

“It’s unfortunate because this was one of the unilateral steps that the North Koreans were making at the beginning of the negotiation process as sort of a confidence-building measure, and so certainly this does have implications for how the North Koreans are thinking about the negotiation process,” Town said.

Channel News Asia on Wednesday quoted a U.S. government source who said the work performed at Sohae “did not seem particularly alarming, and certainly not on a scale of resuming missile tests that have been suspended since 2017.”

“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.

U.S. National Security Adviser John Bolton reiterated on Tuesday that North Korea will not “get relief from the crushing economic sanctions that have been imposed on them” unless they dismantle their nuclear weapons program and “everything associated with it.”

“We’ll look at ramping those sanctions up, in fact,” Bolton warned.