Washington (CNN) New satellite images show North Korea has made rapid improvements to the infrastructure at its Yongbyon Nuclear Scientific Research Center -- a facility used to produce weapons-grade fissile material, according to an analysis published by 38 North, a prominent North Korea monitoring group.

Captured on June 21, the photos reveal modifications to the site's plutonium production reactor and the construction of several support facilities -- long-planned upgrades that were already underway before North Korean leader Kim Jong Un and US President Donald Trump met in Singapore earlier this month.

When contacted by CNN about 38 North's analysis, the Unification Ministry said they "cannot confirm the report" and are "watching it closely."

The report states that "continued work at the Yongbyon facility should not be seen as having any relationship to North Korea's pledge to denuclearize," but the photos suggest that Pyongyang continues to proceed with business as usual when it comes to maintaining its nuclear sites following the summit.

"No change is actually a pretty significant story ... this is still an active site producing plutonium for North Korea," according to Jeffrey Lewis, a professor at the Middlebury Institute of International Studies.

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