US Secretary of Defense Robert Gates says UN sanctions have hit Iran hard and created a rift between the country's supreme leader and president.
He said Ayatollah Ali Khamenei feared that President Mahmoud Ahmadinejad was "lying" about the impact of sanctions.
The UN has slapped four rounds of economic penalties on Iran over its disputed nuclear weapons programme.
Mr Gates argued against a military strike on Iran, saying it would only unite the country against the West.
Addressing the Wall Street Journal's CEO Council in Washington, he said that military action would only "bring together a divided nation" and make Tehran's weapons programme "deeper and more covert".
Iran has repeatedly denied pursuing atomic weapons. It says its nuclear work is for civilian projects such as energy and medical research.
Mr Gates said that UN sanctions had "bitten much harder" than Iran's leadership had expected, and was causing tensions at the highest levels.
"The information we have is that they've been surprised by the impact of the sanctions," Gates said.
"We even have some evidence that [Ayatollah] Khamenei now is beginning to wonder if [President] Ahmadinejad is lying to him about the impact of the sanctions on the economy," he added.
Publicly, Mr Ahmadinejad has insisted that the measures have had no impact on Iran's economy.
Also on Tuesday, the Iranian air force started a five-day military exercise designed to test Iran's defences against air strikes.
Tehran said it was the biggest exercise of its kind it had ever staged.