Iran is removing 27 surveillance cameras at nuclear facilities, the International Atomic Energy Agency (IAEA) head Rafael Grossi said at a news conference in Vienna on Thursday.

The move is further retaliation for the 35-nation IAEA Board of Governors' Wednesday evening resolution criticizing Iran for failing to explain uranium particles at undeclared sites.

Grossi said the move poses a "serious challenge" to its efforts, warning that in three to four weeks, it would be unable to maintain a "continuity of knowledge" about Iran's program.

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.