The director general of the International Atomic Energy Agency said on Monday his organization could not be sure that Iran's nuclear program did not have military aims.
"The agency is unable to provide credible assurance about the absence of undeclared nuclear material and activities in Iran and therefore to conclude that all nuclear material in Iran is in peaceful activities," Yukiya Amano said, according to a copy of his speech to a closed-door meeting of the IAEA's 35-nation governing board.
Amano said that since late last year, Iran had tripled monthly output of higher-grade enriched uranium.
Amano also outlined his agency's failure in recent months to get Iran to grant access to locations, scientists and documents in order to clear up allegations of nuclear weapons development projects.
"Despite intensive discussions, there was no agreement on a structured approach to resolving these issues," Amano told the country board, stressing that the investigation should be conducted according to the priorities of the IAEA, not of Iran.
The IAEA's board of governors was meeting hours before U.S. President Barack Obama and Prime Minister Benjamin Netanyahu were scheduled to meet in Washington to discuss their stance on Iran.
Amano also highlighted Iran's significant increase in uranium enrichment, a process that can be theoretically used to make bomb material.
Iran denies any such aims and says it needs the technology for fuelling its nuclear reactors.
Many of the 35 countries on the board were expected later this week to echo Amano's call on Iran to cooperate with the IAEA.
But a new resolution condemning the country was unlikely because has Tehran started negotiating with the IAEA about the allegations recently after a standstill of more than three years, diplomats said, and some countries would like to give Iran some more time.
Obama on Sunday pledged that his country would not tolerate development of an Iranian nuclear weapon, but warned that increasing warlike rhetoric was pushing up the price of oil and was thus helping Iran finance its nuclear program.