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Officials in Ukraine say measurements at the site of the Chernobyl nuclear disaster have shown increased radiation levels after Russian troops took control of the former nuclear power plant.

The International Atomic Energy Agency (IAEA) said Friday that Ukrainian officials believe the increased radiation may have been caused by military activity or vehicles “stirring up soil still contaminated from the 1986 incident.”

The IAEA added that the readings, while elevated, still remain “within the operational range.”

“The IAEA assesses that the readings reported by the regulator — of up to 9,46 microSieverts per hour — are low and remain within the operational range measured in the Exclusion Zone since it was established, and therefore do not pose any danger to the public,” the agency wrote Friday.

The IAEA’s main concern, however, is the continued and safe operation of Ukraine’s nuclear facilities amid an attack by Russian military forces.

IAEA Director General Rafael Mariano Grossi added that it was of “vital importance” that the country’s plants not be disturbed, according to Friday’s news release.

Ukraine’s State Nuclear Regulatory Inspectorate (SNRI) first relayed news of Chernobyl’s increased measurements earlier on Friday, after observing higher levels of gamma radiation “at a significant number of observation points.” At the time, the agency said it was “impossible” to determine the exact cause of the radiation because of Russia’s military forces.

In response to SNRI, nuclear watchdog group CRIIRAD, based in France, quickly issued a statement warning that Chernobyl contained several sites which were “reservoirs of radiation” that could easily become more problematic if hit by mortars or shells, or if fires broke out in the area.

“Everything must be done to keep their safety devices in operation,” wrote CRIIRAD, which said it was also working to determine if the elevated levels could have been caused by electromagnetic activity or misrepresented due to cyberattacks.

CRIIRAD also urged that respiratory protections — for any workers allowed to remain on the site — are imperative as the soil contains a “cocktail” of radioactive substances.

As of Friday, the IAEA reported that Ukraine’s nuclear reactors continue to operate safely, though Director General Grossi said he remains “gravely concerned” about the events unfolding in Ukraine.