US Secretary of State Colin Powell says he has seen information which suggests that Iran is seeking to adapt its missiles to carry nuclear warheads.
Iran has increased the range of its Shahab-3 ballistic missile
The intelligence suggests Iran has been "actively working on delivery systems", Mr Powell said on his way to Chile.
His comments follow Iran's agreement to suspend its uranium enrichment activities, after talks with the EU.
Iran insists its nuclear activities are peaceful, but the US says they are part of a secret weapons programme.
"I'm talking about what one does with a warhead," Mr Powell told reporters during a brief stop in Brazil on his way to an international meeting in Chile.
"We are talking about information that says they not only have [the] missiles but information that suggests they are working hard about how to put the two together," he added.
Iran faces a 25 November deadline to comply with an International Atomic Energy Agency (IAEA) resolution.
The nuclear watchdog passed a resolution in September calling on Iran to stop enriching uranium.
Iran insists it has complied with all international inspection demands
Successful uranium enrichment could be seen as a key stage in the development of weapons-grade nuclear material.
On Monday, the IAEA said in a confidential report that Tehran had not diverted declared nuclear materials to make weapons.
But the IAEA said it could not rule out the existence of nuclear materials that had not been declared.
The report follows Sunday's agreement by Iran to suspend its uranium enrichment activities by 22 November, after talks with the UK, France and Germany.
The EU offered Iran increased co-operation on trade and energy in exchange for the freeze.