Iran and weapons of mass destruction.

Iran could produce enough weapons-grade uranium to arm a nuclear bomb within two to four months but would still face serious “engineering challenges”— and much longer delays—before it succeeds in making the other components needed for a functioning warhead, a respected U.S. think tank said Monday.

The Institute for Science and International Security (ISIS) did not make a judgment on whether Iran plans to turn its enrichment capabilities toward weapons making. But in its report Monday, it drew a clear distinction between Tehran’s ability to make the fissile core of a warhead by producing 25 kilograms (55 pounds) of weapons-grade uranium from its lower-enriched stockpiles and the warhead itself, Israel Hayom reported.

“Despite work it may have done in the past,” Iran would need “many additional months to manufacture a nuclear device suitable for underground testing and even longer to make a reliable warhead for a ballistic missile,” the report said.

In addition to its payload of weapons-grade uranium, a nuclear warhead also needs to have a complicated trigger mechanism that sets off a chain reaction in the weapons grade uranium—the fissile core of such a weapon—resulting in the high-power blast and widespread radiation characteristic of such weapons. The International Atomic Energy Agency (IAEA) says Iran may have worked secretly on testing such a nuclear trigger.