Iran does not appear to be receiving as much help as would likely be necessary, notably from China or Russia, to reach that goal, according to the 66-page report dated Thursday.

It is also increasingly tough for Tehran to obtain certain critical components and materials because of international sanctions related to its disputed nuclear program.

In addition, Iran has not demonstrated the kind of flight test program generally deemed necessary to produce an ICBM, said the study by Steven Hildreth, a specialist in missile defense who consulted seven external expert reviewers.