A comprehensive new U.S. intelligence report concludes that Iran has resumed research on key components for a nuclear weapon, but that the slow and scattered nature of the effort reflects renewed debate within the regime over whether to build a bomb, U.S. officials said.

The finding represents a significant, if subtle, shift from the main conclusion of a controversial 2007 estimate that Iran had halted its weaponization work.

In finding that Iran has again begun taking steps toward designing a nuclear warhead, the new estimate is likely to be seen as erasing doubt that the earlier document created about Iran's intent.

But the new report reaches no firm conclusions about when Iran might acquire the bomb. The classified estimate has already triggered debate among American officials over whether Iran's apparent hesitation is the result of U.S.-backed sanctions meant to derail any weapons program.

Overall, the National Intelligence Estimate concludes that Iran is conducting "early stage R&D work on aspects of the manufacturing process for a nuclear weapon," said a U.S. official familiar with the report. At the same time, the estimate describes "serious debate within the Iranian regime . . . on how to proceed."

Anticipation surrounding the new estimate has been intense, both because it addresses one of the central national security dilemmas confronting President Obama, but also because critics regarded the previous estimate as confusing, and blamed it for undermining then-President George W. Bush's efforts to ratchet up diplomatic pressure on Iran.

The report carries particular weight because it represents the consensus view of the entire U.S. intelligence community, rather than the assessments of a lone agency.

U.S. officials have said that, unlike the estimate of four years ago, the new one will remain classified and out of public view; they would describe it only on the condition of anonymity. A Wall Street Journal article described aspects of the estimate this week.

Director of National Intelligence James Clapper summarized key points in testimony before the Senate intelligence committee Wednesday, telling lawmakers that Iran's "technical advancement, particularly in uranium enrichment, strengthens our assessment that Iran has the scientific, technical and industrial capacity to eventually produce nuclear weapons."

"Iran is technically capable of producing enough highly enriched uranium for a weapon in the next few years, if it chooses to do so," Clapper said. Whether such a decision had been made, he said, remains unclear.

The new assessment does not entirely refute the 2007 report's most controversial finding, which held that Iran's leaders had halted nuclear weaponization research in 2003, even while pushing forward on uranium enrichment that is regarded as the most difficult step to building a bomb.

U.S. spy agencies remain convinced that Iranian officials ordered a temporary halt to certain military research projects aimed at mastering the complex engineering involved in building nuclear warheads. The stoppage was described in computer notes and files surreptitiously obtained by the United States.