Iran would need at least two years to produce its first nuclear weapon but is not involved in an all-out "crash effort" to build a bomb, a respected think tank said Thursday.

In a new report analysing the available evidence, the International Institute for Strategic Studies (IISS) said it believed the timescale should give more time to international efforts to negotiate a solution.

The report by the London-based body said Iran's nuclear capability had been growing "inexorably" for 25 years and its claim to be pursuing the programme purely for civilian energy purposes was "not credible".

But "the endeavour has not been a crash effort akin to America's Manhattan Project, which produced two kinds of nuclear weapons in three and a half years, or Pakistan's nuclear bomb project, which reached the nuclear-weapons threshold about 11 years after launching an enrichment programme," it said.

The report added: "If Iran wanted to produce the fissile material for a weapon as soon as possible, it could have moved more quickly."

However, it notes that: "Overall, Iran's leaders have tried to keep their presumed weapons intentions ambiguous."

IISS director general and chief executive John Chipman said the analysis, conducted by former US State Department official and non-proliferation expert Mark Fitzpatrick, backed up "the more relaxed timelines" voiced recently in the United States and Israel.

And he stressed: "The timescale is significant because the likelihood of detection allows time for a negotiated solution."

The report added: "If it (Iran) does decide to build nuclear weapons, this would likely be detected before it assembled a single weapon, much less the small arsenal that would be needed to make the risk worthwhile.

"An Iranian nuclear weapon is therefore not inevitable."

Last month, Israel's military intelligence chief, Brigadier General Aviv Kochavi, said Iran was not currently working on producing a nuclear weapon but could make one within "a year or two" of taking such a decision.

Recently, several senior Israeli officials, including the former head of the Mossad intelligence agency, Meir Dagan, have said Iran is unlikely to acquire nuclear weapons before 2015.

Last June, CIA director Leon Panetta warned Iran had enough low-enriched uranium for two weapons, but would need a year to enrich it fully to produce a bomb and another year to develop the weapons system needed to make it viable.