Iran has the capacity to enrich enough weapons-grade uranium for a nuclear warhead in two to four months’ time, a think tank reported Monday.

But the Institute for Science and International Security (ISIS) said it would take “many additional months” for Iran to manufacture a nuclear device suitable for underground testing, and even longer to make a warhead for a ballistic missile.

“If Iran were to attempt to make a nuclear weapon, it would likely face new engineering challenges, despite work it may have done in the past,” the ISIS report said.

ISIS said Iran’s breakout time to produce weapons-grade uranium for a bomb is decreasing as it continues to step up production of 20 percent low-enriched uranium and build up its underground Fordo plant.

Still, the group argues Iran is unlikely to take the step to produce the higher-grade uranium next year because the production would almost certainly be detected by the United States and other countries.