Military strikes against Iran could speed Tehran's development of nuclear weapons, according to a UK think tank.
Iran is expanding uranium enrichment, experts say
A report by the Oxford Research Group says military action could lead Iran to change the nature of its programme and quickly build a few nuclear arms.
Iran denies Western claims it is trying to build weapons, saying its nuclear programme is entirely peaceful.
The study comes as the UN nuclear watchdog is set to discuss the nuclear programmes of Iran and North Korea.
In February, Iran ignored a deadline set by the UN Security Council to stop enriching uranium.
A report by the International Atomic Energy Agency (IAEA) said Iran was instead expanding the programme.
Enriched uranium is used as fuel for nuclear reactors, but highly enriched uranium can be used to make nuclear bombs.
Western powers have threatened to expand sanctions on Iran. These could include travel bans on Iranian officials associated with nuclear and missile programmes.
The US has not ruled out using force but says it wants to give diplomacy a chance.
The Oxford Research Group report is written by nuclear scientist and arms expert Frank Barnaby.
"If Iran is moving towards a nuclear weapons capacity it is doing so relatively slowly, most estimates put it at least five years away," he says.
POSSIBLE NEXT STEPS
New UN resolution on tougher economic sanctions, tabled by US or European allies
US pressure on Europeans to step up bilateral sanctions
New initiative to get Iran back to talks
Mr Barnaby adds that an attack on Iranian nuclear facilities "would almost certainly lead to a fast-track programme to develop a small number of nuclear devices as quickly as possible".
He says it "would be a bit like deciding to build a car from spare parts instead of building the entire car factory".
The BBC's diplomatic correspondent Jonathan Marcus says that with two US navy aircraft carrier strike groups in the Gulf region and US spokesmen refusing to rule out force, this study is timely and highlights what most air power experts have been saying for some time.
An operation to destroy Iran's nuclear facilities would be neither brief nor limited in scope, our correspondent says. Multiple targets would have to be hit, and the outcome would be far from clear, especially if Iran has hidden facilities unknown to US intelligence.
But he points out that this is not a military study - written by a noted atomic scientist and peace campaigner, it looks more at the aftermath of a potential US attack and questions the central rationale for any military operation.
On Monday the IAEA board of governors is due to discuss both Iran and North Korea.
The BBC's Bethany Bell in Vienna says that while there is little progress on the Iranian nuclear file there has been movement on North Korea.
Last month Pyongyang agreed to take the first steps towards nuclear disarmament, as part of a deal reached during talks in Beijing.
Under the agreement, North Korea promised to shut down its main nuclear reactor in return for fuel aid.