Defense Secretary Gates said Tehran was not close to having a bomb. A new report in which senior advisers to Obama participated urges further sanctions.
He also singles out Iran's pursuit of a nuclear program as endangering international security.
An IAEA team visits the site of an alleged nuclear weapons facility bombed by Israel in September.
As UN nuclear watchdog meets in Vienna, sparring is sharp over Iran's goals for its program.
The US is pushing stronger action, but others want only mild measures.