Mullen said he doesn't believe that Iran's nuclear plant is for civilian use "for a second."
"In fact, the information and intelligence that I've seen speak very specifically to the contrary. Iran is still very much on a path to be able to develop nuclear weapons, including weaponizing them, putting them on a missile and being able to use them," he said.
Mullen said the United States was focused on dialogue and engagement about Iran's nuclear ambitions, but "in a realistic way."
"They've got a history of gamesmanship that certainly doesn't include closing on significant steps to indicate to the international community that they're not doing this," he said.
On Saturday, Iran said its first atomic power plant built by Russia in Bushehr had begun operations, ahead of a new round of talks with Western powers over the country's controversial nuclear drive.
"Without any propaganda and fuss we sealed the cover of the reactor and all the fuel rods are in the core of the reactor," atomic chief Ali Akbar Salehi was quoted as saying by Fars news agency, without specifying when.
But Salehi said the authorities in the Islamic republic "hope that electricity produced at the Bushehr plant will be connected to the national grid in a month or two."
Iran says it needs the plant, which had been under construction since the 1970s in the southern port city of Bushehr before it was completed by Russia, to meet growing demand for electricity.
But Western governments suspect Iran's nuclear program masks a drive for an atomic weapons capability, an ambition Tehran has steadfastly denied.