The assessment that Iran was ahead of schedule in its enrichment work is based on new information gathered by Israel and several other western intelligence agencies, according to reports by Israeli newspaper Haaretz.
Citing sources with knowledge of western efforts to gain access to Tehran's nuclear facilities, signs of the advanced state of Iran's nuclear program began to emerge in February.
That month, Iranian officials denied inspectors from the International Atomic Energy Agency access to its nuclear facility in Parchin — where western officials believe the bulk of Iran's nuclear weapons work is taking place.
In July, European media reports claimed leaders of Iran's Revolutionary Guards had sent a team of nuclear scientists to assist with ongoing enrichment work on the country's nuclear facility in Lavizan, miles from Tehran.
At the Lavizan location, Iranian officials are suspected of developing nuclear warheads and detonators, according to those reports. IAEA inspectors visited the Lavizan base, in 2006. The military installation is part of the Iranian military's missile development agency, according to Haaretz.
Tehran has repeatedly claimed its nuclear work is strictly focused toward developing a new energy source for the country. However, western powers continue to argue Iran is actively pursing a nuclear weapon, citing the country's refusal to allow international inspectors access to its facilities.
Three subsequent meetings between Iranian diplomats and members of the P5+1 council — the five permanent United Nations Security Council members and Germany — have yielded little progress in shedding any light into Tehran's nuclear work.
Israeli Prime Minister Benjamin Netanyahu has said that Israel will not hesitate to launch an preemptive attack against Iran's nuclear facilities, should it become clear Iran is pursuing nuclear weapons.
Officials from presumptive GOP presidential nominee Mitt Romney's campaign said in July that the former Massachusetts governor would support Israel if it decided to launch an attack against Iran.
For its part, the Obama administration has repeatedly stated that military action remains an option, but has focused its efforts on sanctions and talks to convince the country's leaders to back off their ongoing nuclear work.
That said, U.S. military and intelligence officials have said publicly there has been no evidence that Iran has crossed any of the "red lines" that would prompt military action by American forces.
These so-called "red lines" are U.S. or Israeli-imposed limits on how far Iran can go in terms of advancing their nuclear program. Should Iran cross any one of these red lines, it could trigger an armed response by either Washington or Tel Aviv.