Iran is still several years away from acquiring a nuclear weapons capability, according to a study published by an influential London-based think tank.
Iran says its nuclear programme is only for peaceful purposes
The International Institute for Strategic Studies has assessed Iran's nuclear, chemical, biological and long-range missile activities.
It says a diplomatic showdown with the European Union and the United States could be inevitable.
Iran's political restraint thus far may not last, the report's authors say.
One of them, Dr Gary Samore, told the BBC that it might take five years for Iran to overcome all the technical difficulties to produce a nuclear weapon.
But given Tehran's cautious behaviour so far, a decision on whether to build such a capability may be much further away.
"They're trying to avoid international reaction and I think it's perhaps more likely that they would try to develop their nuclear capabilities over a much longer period of time, a decade or 15 years," he said.
The assessment from the IISS comes two weeks before a meeting in Vienna to discuss Iran's nuclear ambitions.
"Iran's Strategic Weapons Programmes - A Net Assessment" charts the political history and progress of Iran's nuclear programmes since its origins under the Shah in the late 1950s.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is converted into a gas by heating it to about 64C (147F)
Gas is fed through centrifuges, where its isotopes separate and process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
BBC diplomatic correspondent Jonathan Marcus says what matters to arms control experts is how far advanced Iran is in its quest to be able to produce and enrich uranium.
Iran says it wants this capability to provide fuel for power-generating reactors - but the same technology could also provide the fissile material for a bomb.
The report says Iran faces two great technical hurdles before it can have a nuclear weapons capability:
- producing sufficient fissile material
- building a functioning warhead.
But it does not attempt to provide an assessment of the crucial issue of Iran's political intentions.
The report sheds little light on Iran's potential chemical and biological weapons programmes.
But it does give considerable detail on its long-range missile programme, where it says there have been considerable technical advances in recent years.
Iran seems to be focusing on fielding more of its Shahab-3 systems, a variant of a North Korean missile, capable of hitting targets in Israel, much of Turkey and southern Russia.
Tehran has acknowledged a long history of undeclared nuclear work.
So the report's authors are not optimistic that Iran's acceptance of temporary restrictions on its nuclear activities will continue, our correspondent says.