Ukrainian authorities say the decommissioned Chernobyl nuclear plant, the site of the world’s worst nuclear disaster, has been knocked off the power grid. Emergency generators are now supplying backup power.

The state communications agency says the outage could put systems for cooling nuclear material at risk.

The cause of the damage to the power line serving Chernobyl was not immediately clear, but it comes amid Russia’s invasion of Ukraine. The site has been under control of Russian troops since last week.

Ukrainian grid operator Ukrenerho said that according to the national nuclear regulator, all Chernobyl facilities are without power and the diesel generators have fuel for 48 hours. Without power the “parameters of nuclear and radiation safety” cannot be controlled, it said.

Foreign Minister Dmytro Kuleba said the grid supplying electricity is damaged and called for a cease-fire to allow for repairs.