President Donald Trump warned the USA military was "locked and loaded, should North Korea act unwisely" and that the United States would unleash "fire and fury" on the North if it continued to threaten America.
U.S. concerns about the threat from North Korean missiles spiked past year as North Korea test-fired long-range missile over Japan and threatened to launch ballistic missiles toward the Guam, a major U.S. military hub in the Pacific.
During an earlier period of outreach with the West a decade ago, North Korea invited worldwide journalists in 2008 to film the destruction of the cooling tower at the Yongbyon nuclear reactor, from which it had been harvesting plutonium to make its first bombs.
But as if to reconfirm, Pentagon spokeswoman Dana White said Monday, that US defense officials would only speak to military-related negotiations on North Korea and that there was "no specific timeline" for the diplomatic process.
As such, preplanned improvements like this are expected to take place in the meantime.
North Korea is carrying out rapid improvements to its nuclear research facility, a monitor said on Wednesday, despite declaring a commitment to denuclearisation of the peninsula at the Singapore summit.
The 38 North report said the imagery points to work "continuing at a rapid pace" in recent months at the site, which contains a plutonium production reactor and an experimental light-water reactor for possible energy production.
The Yongbyon facility is the only known nuclear reactor used for North Korea's weapons program, according to The Guardian.
The light water reactor is created to provide electricity for civilian purposes but could also produce fissile material for nuclear weapons.
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38 North warned that the images alone were not proof that North Korea was going against the denuclearization pledge.
The improvements to the nuclear reactor include a water channel that leads to a new cooling water pump house.
The 38 North analysts caution that the satellite images "should not be seen as having any relationship to North Korea's pledge to denuclearize", as Time Magazine reports.
But the report concluded that operations appear to be moving ahead at a uranium enrichment plant because of "roof staining" that suggests "water vapor coming from the six cooling units associated with gas centrifuge operations".
But in addition to the newly completed upgrade to the North Korean reactor's cooling system, the new satellite photos show that two new industrial buildings and an office building for nuclear engineers have also now been constructed at the site, as The Daily Beast reported.
Coal bins near the site's thermal plant - where heat could be used to generate steam to drive the nuclear reactor - also appeared in the images to have been depleted since 38 North examined photographs in early May. It said "there was is no visible evidence yet" to suggest that operations are underway at the experimental reactor.
The images also showed multiple trucks near the site's radiochemical lab, which further suggests recent activity at the facility.
The 38 North report says that, from the satellite images, whether or not the North Korean reactor is now active is impossible to determine.
"The North's nuclear cadre can be expected to proceed with business as usual until specific orders are issued from Pyongyang", said the analysis by experts Frank Pabian, Joseph Bermudez Jr. and Jack Liu.