More than a month after taking over the Chernobyl nuclear power plant, Russian troops have left the site, giving control back to local officials, Ukrainian authorities said early Friday.

The world’s worst-ever nuclear disaster took place at Chernobyl in 1986, and Ukraine’s state power company, Energoatom, said the Russian troops were exposed to “significant doses” of radiation while digging trenches around the plant. The Associated Press notes there has not been independent confirmation of this claim.

Russia has said it is scaling back its military operations and withdrawing forces north of Kyiv, but the United States and other Western countries believe they are using this time instead to regroup and get much-needed supplies to troops. In his nightly video address on Thursday, Ukrainian President Volodymyr Zelensky warned that he also thinks this is just a military tactic.

“We know their intentions,” Zelensky said. “We know that they are moving away from those areas where we hit them in order to focus on other, very important ones where it may be difficult for us. There will be battles ahead.”