The head of the U.N.'s nuclear watchdog agency said Thursday that Iran is removing 27 surveillance cameras from nuclear sites in the country.

Rafael Mariano Grossi, the director-general of the International Atomic Energy Agency (IAEA), made the comments at a suddenly called news conference in Vienna.

Grossi said the move poses a "serious challenge" to its efforts. Iran did not immediately acknowledge it.

Grossi said that would leave "40-something" cameras still in Iran. The sites that would see cameras removed include its underground Natanz nuclear enrichment facility, as well as its facility in Isfahan, Grossi said.

On Wednesday, Iran said it shut off two devices the IAEA uses to monitor enrichment at Natanz. Iranian officials also threatened to take more steps amid a yearslong crisis that threatens to widen into further attacks.

That came ahead of a vote before the IAEA's board censuring Iran over what the agency calls Iran's failure to provide "credible information" over man-made nuclear material found at three undeclared sites in the country.