Asked at a senate hearing about Tehran's missile capability, James Miller, principal deputy undersecretary of defense for policy, said current estimates indicate "that it could potentially be as soon as 2015."
But he said that estimate assumed "foreign assistance" to enable Iran to improve its missile technology.
A report last year from the US Air Force National Air and Space Intelligence Center said Iran could build an intercontinental ballistic missile that could hit US soil by 2015-2018, if it received outside help.
Analysts say Iran's Safir space launch vehicle, which Tehran put into orbit in February 2009, has the potential to be converted into a long-range missile.
Washington closely follows Iran's missile program and has cited threats from Tehran and North Korea as the main impetus for building up missile defenses for the United States and allies.
The US administration also accuses Tehran of a clandestine effort to build nuclear weapons.
Miller confirmed previous estimates that it would take "well beyond a year" for Iran to develop a nuclear weapon and "more likely in that plus three-year time frame."
Last week, General James Cartwright, vice chairman of the US Joint Chiefs of Staff, told a senate hearing that Iran could make enough highly enriched uranium for a nuclear bomb in one year but would most likely not be able to field a usable weapon for three to five years.
He also cautioned often impatient lawmakers that a limited US military strike was not likely to be "decisive" in halting Tehran's disputed nuclear program.
In a related development Brigadier General Hossein Salami, deputy chief of the Guards, told state-owned Al-Alam television channel the Revolutionary Guards will begin Thursday a three-day military drill which will test a range of home-built missiles and other weapons.
The exercise will see ground, air and naval units of the Guards participating and is aimed at "preserving the security of Persian Gulf, Strait of Hormuz and the Gulf of Oman," Brigadier General Hossein Salami, deputy chief of Guards told state-owned Al-Alam television channel.
"Iranian missiles and other weapons will be used … to test Iran's defence capability," Salami said.
