Iran's first nuclear power plant has stepped up operations after years of delays, according to Iranian state television.
Celebrations for the official launch of the Bushehr plant kicked off Monday, with senior Iranian and Russian officials in attendance.
Meanwhile, International Atomic Energy Agency chief Yukiya Amano said he was "increasingly concerned" about new intelligence that Tehran may be experimenting with nuclear warheads.
"Iran is not providing the necessary cooperation to enable the agency to provide credible assurance about the absence of undeclared nuclear material and activities in Iran," Amano told the 35-nation board.
The U.S. and its allies accuse Iran of using its civilian nuclear program to develop nuclear weapons, a claim Iran has denied.