"We think they do, quite frankly," Mullen said when asked if Iran had enough nuclear material to manufacture an atomic bomb.
"And Iran having a nuclear weapon, I've believed for a long time, is a very, very bad outcome for the region and for the world," said Mullen, the chairman of the US Joint Chiefs of Staff.
While the United States and European allies have expressed concern previously that Iran could soon have enough enriched uranium to manufacture a nuclear weapon, Mullen's more definitive comments went a step further.
The admiral's remarks came in the wake of a report by the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), that said Tehran had made strides in its uranium enrichment work.
Some analysts have said Iran may soon have sufficient material to make a nuclear bomb.
According to the IAEA, Tehran now has 1,010 kilogrammes of low-enriched uranium hexafluoride from its enrichment activities at a plant at Natanz.
That "is sufficient for a nuclear weapons breakout capability," according to David Albright, president of the Washington-based Institute for Science and International Security and an expert on Iran's nuclear program.
A breakout capability is when there is sufficient low-enriched uranium, which is used for nuclear fuel, to turn into highly-enriched uranium needed for nuclear weapons.
Iran denies its atomic work is designed to build a nuclear arsenal and says it wants to develop nuclear technology to generate electricity for a growing population.
