Iran could produce enough weapons-grade uranium to make an atomic bomb within two to four months and then would need an additional eight to 10 months to build the device, experts said Monday.

The authors of a new report on Iran's nuclear program say Tehran has made progress in its uranium enrichment effort but that the United States and UN weapons inspectors would be able to detect any attempt at a "breakout" -- at least for the moment.

The report, released by the Institute for Science and International Security, offers estimates on uranium stockpiles and enrichment rates based on figures from inspections of Iran's program by the UN watchdog, the International Atomic Energy Agency (IAEA).

To amass the 25 kilograms of highly-enriched uranium needed for one nuclear weapon, Iran "would require at least 2-4 months," the report.

To reach that goal, Iran would have to draw on its uranium enriched to 3.5 percent as well as stocks of 20 percent enriched uranium, it said.

The report appears roughly in line with the US government's view that once Iran made a decision to make a bomb, Tehran could be months away from generating sufficient amounts of weapons-grade material and then additional months would be required to construct a device.

The findings confirm comments made to AFP last month by one of the authors, David Albright, a leading expert on Iran's nuclear project.

Once Iran had generated enough highly-enriched uranium, it could take about eight to 10 months to construct a nuclear weapon, Albright told AFP on Monday.

Defense Secretary Leon Panetta said on September 11 that the United States would have about a year to take action if Iran decided to build a nuclear weapon.

The time needed for Iran to quit the nuclear non-proliferation treaty and "dash" or "break out" to make the bomb would give the United States and its allies time to react if necessary, the report said.

"Although Iran's breakout times are shortening, an Iranian breakout in the next year could not escape detection by the IAEA or the United States.

"Furthermore, the United States and its allies maintain the ability to respond forcefully to any Iranian decision to break out. During the next year or so, breakout times at Natanz and Fordow (facilities) appear long enough to make an Iranian decision to break out risky," it said.

But as Iran's supply of 20 percent enriched uranium increases, the time needed to produce enough material for a bomb or bombs will decrease, it said.

Iran's expanding network of centrifuges could make it increasingly difficult for inspectors to spot Tehran's progress, it said.

"Iran may be seeking the ability to produce sufficient WGU (weapons grade uranium) faster than the IAEA inspectors could detect it," the authors wrote.

Despite repeated accusations from Western countries and critical findings from UN inspectors, Iran insists its nuclear program is designed purely for peaceful purposes.

The United States is under pressure from Israel to set a precise deadline for military action but prefers for now to pursue a course of ever tighter sanctions to try to force Tehran to the negotiating table.

Iran denies it is seeking atomic weapons, maintaining that its nuclear program is for civilian energy purposes.

Once Iran possesses enough weapons-grade material for a bomb, it would be extremely difficult for UN monitors or outside countries to determine if Tehran had built a nuclear device, the report said.

"If Iran successfully produced enough WGU for a nuclear weapon, the ensuing weaponization process might not be detectable until Iran tested its nuclear device underground or otherwise revealed its acquisition of nuclear weapons," it said.

"Therefore, the most practical strategy to prevent Iran from obtaining nuclear weapons is to prevent it from accumulating sufficient nuclear explosive material."