New intelligence reveals Iran is working on a nuclear bomb, according to diplomats who shared their insight with The Associated Press.

The intelligence shows the country has been trying to calculate the destructive power of a nuclear warhead, another step toward building such a weapon.

The six anonymous diplomats said information from America, Israel, and at least two other countries suggests Iran's research was done in the last three years.

David Albright, who leads the Washington-based Institute for Science and International Security, responded to the latest revelation. The group often advises Congress and other U.S. government branches on issues related to Iran.

"You want to have a theoretical understanding of the working of a nuclear weapon that is then related to the experiments you do on the various components," Albright said. "The two go hand-in-hand."

Iranian leaders call the allegations false. But the International Atomic Energy Agency said it can't disprove the claims unless Iran cooperates with its investigation.

The country has failed to cooperate with IAEA efforts to investigate such claims for more than five years.

Iran Foreign Ministry spokesman Rahmin Mehmanparast told reporters his country will only start answering the agency's "questions and concerns" when "our rights and security issues" are recognized.