By The Telegraph
Asked at a senate hearing about Tehran's missile capability, James Miller, principal deputy undersecretary of defence 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 Centre 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 defences 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.
It comes as envoys of six major powers huddled behind closed doors at China's UN mission in New York for another round of bargaining on new UN sanctions against Iran.
Ambassadors from the five veto-wielding members of the UN Security Council – Britain, China, France, Russia and the United States – plus Germany have been meeting almost daily since last Wednesday but made clear that there would not comment on the content of their discussions.
A diplomat close to the discussions said, however, on condition of anonymity that the Russian side made "some rather constructive proposals" in discussions of a US draft outlining sanctions against Tehran.
The source said the Chinese had yet to give their comments on the draft and it was unclear whether they did so at Tuesday's meeting.
The package, already endorsed by Washington's European allies, would include a full arms embargo, a ban on new investments in Iran's energy sector, restrictions on shipping and finance as well sanctions targeting the business interests of the Islamic republic's powerful Revolutionary Guards, sources said.
Diplomats say they expect weeks of hard-nosed bargaining before a text, likely to be toned down to make it palatable to the Chinese and the Russians, can be brought to a vote by the full 15-member Security Council.
The council has already imposed three sets of sanctions on Iran over its refusal to halt its uranium enrichment program, which the West sees as a cover to acquire a nuclear weapons capability.
Tehran however maintains its nuclear program is peaceful and insists that it is entitled to conduct nuclear enrichment under the nuclear Non-Proliferation Treaty (NPT) it has signed.
The issue is expected to figure prominently next month when a NPT review conference is held at UN headquarters.