Photo/Illutration This composite of satellite images taken by Planet Labs PBC shows smoke rising from fires at the Zaporizhzhia nuclear power plant in southeastern Ukraine on Aug. 25. (Planet Labs PBC via AP)

BERLIN--The head of the International Atomic Energy Agency said Monday that the U.N. nuclear watchdog’s long-awaited expert mission to the Zaporizhzhia power plant in Ukraine “is now on its way.”

IAEA director general Rafael Grossi has for months sought access to the Zaporizhzhia plant, Europe’s biggest, which has been occupied by Russian forces and run by Ukrainian workers since the early days of the 6-month-old war.

The urgency has been heightened in recent days as Russia and Ukraine have traded claims of strikes at or near the plant, intensifying fears that the fighting could cause a massive radiation leak. Last week, the facility was temporarily knocked offline.

“The day has come,” Grossi wrote on Twitter, adding that the Vienna-based IAEA’s “Support and Assistance Mission ... is now on its way.”

“We must protect the safety and security of #Ukraine’s and Europe’s biggest nuclear facility,” he wrote. “Proud to lead this mission which will be in #ZNPP later this week.” Grossi, who didn’t provide a more precise timeline or give further details, posted a picture of himself with 13 other experts.

Ukraine has alleged that Russia is essentially holding the plant hostage, storing weapons there and launching attacks from around it, while Moscow accuses Ukraine of recklessly firing on the facility. The Zaporizhzhia plant has six reactors.

The IAEA tweeted that the mission will assess physical damage to the facility, “determine functionality of safety & security systems” and evaluate staff conditions, among other things.