The International Atomic Energy Agency has for years said that, because it cannot be certain there are no undeclared nuclear sites or material in Iran, it cannot make such a conclusion.

“I cannot say at this stage how long it will take (to make this assessment) but it is a matter of years at least. Not months, not weeks, but years. Years and years,” IAEA chief Yukiya Amano told reporters.

Amano said reaching this “broader conclusion” on Iran’s nuclear programme depended in large part on Tehran implementing the Additional Protocol of the Nuclear Non-Proliferation Treaty, as some 120 countries have done.

Under a framework nuclear deal agreed by Iran and six world powers in April that is meant to be finalised by the end of this month, Iran undertook to provisionally apply the protocol.

Doing so will oblige Iran to submit a “very detailed declaration” of its nuclear facilities and to update this every three months, Amano said.

The protocol also gives the IAEA the right to request access to sites — including military ones — where it suspects the presence of nuclear material that has not been declared to the agency.

The “broader conclusion” would also require progress in the IAEA’s stalled investigation into the “possible military dimensions” (PMD) of Iran’s nuclear program.

The probe concerns allegations that prior to 2003, and possibly since, Iran conducted research into developing a nuclear weapon.

Tehran rejects the claims, saying its program is and has always been peaceful. The IAEA’s investigation has been stalled since August.

Earlier on Monday Amano, 68, urged Iran to cooperate more in the probe, which Western powers say is a vital part of the mooted final deal.

Amano said that if Iran cooperated, closing the “PMD” investigation would take less time than arriving at the “broader conclusion” about the peaceful nature of the programme.

“Clarification of PMD is not an endless process. It cannot be done in weeks… but it will not take years,” the veteran Japanese diplomat said.

Diplomats and experts from Iran and the P5+1 are negotiating intensively to nail down the final deal by the June 30 target date.

It would see Iran scale down its nuclear program in order to make any dash to make a bomb virtually impossible. In exchange painful Western sanctions would be lifted.