A U.S.-based think tank has revealed satellite imagery over North Korea that suggests Pyongyang may be renovating its Yongbyon uranium enrichment facility.

Images released by the Middlebury Institute of International Studies at Monterey(MIIS) dated Tuesday show that trees that stood in the area in early August were uprooted a month later. Six big holes were dug and a tall side wall was built.

The institute speculated that the North was in the process of building additional centrifuges used to produce highly enriched uranium at the facility.

According to CNN, the White House, Pentagon and U.S. intelligence agencies all declined to comment on the latest observations.

The International Atomic Energy Agency(IAEA) recently released a report, saying North Korea appears to have resumed operations at the Yongbyon complex. The IAEA called it a reason for serious concern and deeply troubling.