US lawmakers said the revelation warranted an immediate and severe response.
"The Iranians will have a nuclear weapon if something doesn't change their minds. We need tough sanctions. We need to do them now," Democratic Senator Evan Bayh told "Fox News Sunday."
Republican Senator Lindsey Graham, a leading congressional voice on defense and security matters, said he would like Congress to set aside a week dedicated to a raft of new sanctions legislation against the Islamic republic.
"Let's have Iran week in the Senate and get something done," he said, proposing to discuss a series of measures "that would empower the president and our country to be tough and to put actions behind words."
But a top White House adviser on Sunday downplayed the report, telling US television that Washington would "stand by the reports that we've put out," regarding Iran's nuclear ability.
"Whether they know how to do it or not is a matter of some conjecture," Jones said.
Lawmakers said they still supported US initiatives to engage Tehran in talks, but were losing faith in the prospect that diplomacy alone will force Iran to abandon its alleged nuclear weapons program.
"Having this dialogue is good, but you've got to hold them to their word. What matters ultimately is not what they say, but what they do," said Bayh.
"I'm afraid they're running the clock on us," he said, pointing to "a real sense of urgency" in the nuclear standoff.
Meanwhile, Washington's ambassador to the United Nations Susan Rice stressed Sunday that six world powers collectively known as the P5 + 1 were in "intense negotiations" with Tehran over its controversial nuclear drive, calling recent diplomatic inroads "only a beginning."
"The onus is now squarely on Iran to adhere to the commitments it has made," Rice told NBC television.
The Times report followed recent revelations that Iran secretly built a second uranium enrichment plant near the holy city of Qom, which further raised suspicions about the aim of its controversial nuclear program.
The report described a complex program apparently launched in 2002 and run by the Iranian Defense Ministry that seeks to develop "a nuclear payload to be delivered using the Shahab 3 (medium-range) missile system."
IAEA chief Mohamed ElBaradei earlier said that agency experts will inspect the Qom plan on October 25 as he praised Tehran's shift "from conspiracy to cooperation" while warning that "concerns" remain over its nuclear aims.
"They have a pattern of deception, a pattern of breaking agreements they agree to," Bayh said. "They respect strength and strength alone."
Other lawmakers echoed limited confidence in the negotiation process and suggested Iran could not be trusted to keep its word.
Republican Senator Saxby Chambliss said talks with Iran would yield more results with sanctions already in place.
"Let's impose sanctions, let's get our allies together and say this is what we're going to do," he said.
The Iranian regime could then be warned that it needs to "come clean" on its weapons program, the conservative lawmaker added. Continuing dialogue with Tehran, he said, would not achieve results.
ElBaradei however, insisted Sunday that he feels "Iran's case can be solved through dialogue."
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