Ukraine said on Sunday it condemned a "breach of territorial sovereignty" by employees of the U.N. nuclear watchdog who visited the Russia-held Zaporizhzhia nuclear power plant in Ukraine via Russian-occupied territory.

The territories through which the latest unauthorized rotation of IAEA experts at the Zaporizhzhia NPP took place are the territories of Ukraine temporarily occupied by Russia, and they will always remain Ukrainian.

International atomic monitors are for the first time traveling through Russian territory rather than Ukraine to reach Europe’s biggest nuclear-power plant, said three diplomats briefed on the matter — a blow to Kyiv’s desire to avoid international recognition of the Kremlin’s ownership claims.

A new monitoring mission from the U.N. nuclear watchdog arrived on Saturday at the Russian-held Zaporizhzhia nuclear power plant in Ukraine for the first time through Russian territory, a Russia-installed head of the plant said.