State Department spokesman Philip Crowley's remarks appear to give credence to a report in The Times on Monday saying it had obtained notes describing a four-year plan by Iran to test a neutron initiator, or bomb trigger.
"There's been a public report about an issue related to… Iran's nuclear program. It's safe to say the United States government will be investigating those reports," Crowley told reporters.
The Times reported that foreign intelligence agencies dated the documents to early 2007 — four years after US agencies had assessed Iran had suspended efforts to produce nuclear weapons.
It said the documents detailed a plan to test whether the device works — without leaving traces of uranium that the outside world could detect.
Iran insists its nuclear program is solely for civilian purposes and rejects Western suspicions that it is covertly trying to develop a bomb.
In Tehran, Iranian foreign ministry spokesman Ramin Mehmanparast dismissed the Times claim as a "scenario" hatched by Western powers.
Crowley was initially reluctant to comment on the news report, but later raised it in the context of growing concerns about Iran's failure to respond to international inquiries about its suspect nuclear program.
He said the discovery of a new uranium enrichment site near Qom, Iran's plans for other facilities and the "revelation this week about nuclear triggers… all adds up to the fact that Iran has yet to really come to… the international community and address our concerns in a meaningful way."
When asked to elaborate on the revelation of a nuclear trigger, Crowley, the assistant secretary of state for public affairs, said: "I will not discuss intelligence matters."
But when pressed further, he said the US government will investigate the report.
And when asked to give his opinion of the news report, Crowley first replied "no" before adding "it was a fine piece of journalism. Enough said."
A senior State Department official told reporters he is "sure somebody in the government is looking into the issue," but he did not know whether the documents in the Times report had been authenticated.
The US-based Institute for International Science and Security (ISIS) said the documents, which it said The Times showed its experts before publishing the report, could be a "smoking gun" showing proof of what the Iranians are up to.
"But ISIS urges caution and further assessment of this document, in particular to confirm the document's date and with how the document fits with other information regarding Iran's nuclear weaponization activities both prior to 2003 and any work afterwards," it said on its website.
With a year-end deadline, President Barack Obama's administration has signalled in the last few days that time is running out for Iran to seize its offer of diplomatic engagement for resolving nuclear and other issues.
Secretary of State Hillary Clinton and other officials have renewed talk of imposing a fourth round of UN Security Council sanctions on Iran.
