Countries differ in their assessment of Iran's secret nuclear facility which is driving how leaders may address Iran's nuclear intent, U.S. officials said.

Leaders of the United States, France and Britain announced during last week's Group of 20 economic summit in Pittsburgh that they had evidence that Iran had been building a nuclear facility near Qom well in advance of the country's recent notification of such activity to the International Atomic Energy Agency.

Some nuclear experts theorize that the Qom facility is part of something larger, The New York Times reported Tuesday. However, a senior U.S. official knowledgeable about the relevant intelligence told the Times he thought the secret plant was "the big one," while cautioning that Iran is "a big country."

Iranian officials are to meet this week in Geneva with permanent members of the U.N. Security Council -- China, Britain, France, Russia and the United States -- plus Germany to discuss its nuclear aspirations.

Some Israeli and European officials said the United States is being overly cautious, which American intelligence officials deny, the Times reported.

Rolf Mowatt-Larssen, the former head of intelligence at the U.S. Energy Department and a nuclear expert who worked for the CIA, told the Times the apparent differences of opinion could simply be how information is interpreted, what he called "tradecraft."

"It's often tradecraft that gets us bollixed up," he said. "It comes down to interpreting the same data in different ways, in looking at the same information and coming up with different conclusions."