Earlier in the day, the Atomic Energy Organization of Iran said that uranium enrichment at the Fordow Fuel Enrichment Plant had reached 20 percent.
"We express our serious concerns. We urge Iran to observe the provisions of the nuclear agreement and refrain from actions that could undermine it," Kato said at a press conference.
The maximum level at which Iran could enrich uranium under the Joint Comprehensive Plan of Action was 3.67 percent. Under a new law passed in December, Tehran is determined to enrich uranium at 20 percent and beyond, as well as deny IAEA inspectors access to Iranian nuclear sites, in a bid to prompt third countries, specifically the United States, to lift economic sanctions.