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.

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.

The UN's nuclear watchdog defended itself on Monday against criticism from the Ukrainian government following the recent rotation of agency experts at the Russian-occupied Zaporizhzhya Nuclear Power Plant.

A new monitoring mission from the U.N. nuclear watchdog arrived on Saturday at the Russian-held Zaporizhzhia nuclear power plant in southeastern Ukraine, Russia's Rosatom company said, after weeks of delay caused by military activity around the site.