The head of the U.N. nuclear agency says he is “increasingly concerned” about new intelligence on Iran's controversial nuclear program.

International Atomic Energy Agency chief Yukiya Amano said Monday, at the start of a five-day board meeting, that the agency is worried about Tehran's possible warhead experiments.

He added that while Iran had shown “greater transparency” than usual, it still was not providing the U.N. agency with the “necessary cooperation” on nuclear matters.

The U.N. Security Council has imposed four sets of sanctions on Tehran for its uranium enrichment activities and lack of cooperation with international inspectors. Iran says its nuclear program is devoted only to the peaceful generation of energy.

For the past nine years, Tehran has failed to provide the U.N.'s nuclear watchdog agency with the requested guarantees that its nuclear program is for peaceful purposes.

During the Vienna meeting, the IAEA board will also discuss concerns about possible nuclear proliferation activities by Syria and North Korea. And it will examine nuclear safety issues, six months after a strong earthquake and tsunami damaged Japan's Fukushima nuclear plant.

Also Monday, Iran celebrated the delayed launch of its Bushehr nuclear power station – the first in the country. Iranian energy officials and their Russians counterparts attended the ceremony.

The plant had missed its late August start-up date due to delays Iranian lawmakers blame on the plant's Russian builders. Last week, Iranian officials said the Bushehr plant had been connected to the national electricity grid with a power of about 60 megawatts.