The UN atomic agency bemoaned on Friday "major differences" with Iran after two fruitless visits probing suspected nuclear weapons work, adding that Tehran had substantially boosted uranium enrichment.
"An intensive discussion was held on the structured approach to the clarification of all outstanding issues related to Iran's nuclear programme" during two recent visits, the International Atomic Energy Agency said.
"No agreement was reached between Iran and the Agency, as major differences existed," it said in a new report circulated to member states late Friday and seen by AFP.
"The agency continues to have serious concerns regarding possible military dimensions to Iran's nuclear programme," it said, two days after a team led by Herman Nackaerts returned from a visit dubbed a "failure" by Washington.
A senior official familiar with the investigation also hit out at Iran's negotiating tactics during the two trips to Tehran in the past month -- another is not planned -- with the IAEA team only able to speak to "middle men."
"We wanted to be sure that we could run this investigative process the way we wanted to run it... (with) the kind of normal things that you would expect in a normal investigation," the official said on condition of anonymity.
"Iran had difficulties with this. Iran wanted to really constrain the process, and put us in a harness, having an exhaustive list of questions and things like that."
In particular, Iran refused to allow the team access to the Parchin military site near Tehran, where a November IAEA report said scientists had conducted suspicious explosives tests.
That extensive report focused on a number of areas where the IAEA believes Iran carried out a range of activities the agency said could only conceivably be aimed at developing nuclear weapons.
Iranian officials repeated their assertion during the visits that the report, which has prompted Western countries to ramp up sanctions and raised speculation of Israeli plans for air strikes, was based on forgeries, the agency said.
"Iran's declaration dismissed the agency's concerns... largely on the grounds that Iran considered them to be based on unfounded allegations," the new report said.
From Tehran, the Islamic republic's envoy to the IAEA, Ali Asghar Soltanieh, said Friday that access to the Parchin site required agreement on the reasons for such a visit.
"There was no agreement, and talks must continue until there is one."
But he insisted: "Iran, which is a responsible country and one that respects international rules, will continue its cooperation with the IAEA."
The IAEA also said that Iran had tripled its capacity to enrich uranium to 20-percent purities since November, and was now producing around 14 kilos uranium per month, with around 105 kilos already stockpiled.
Enriching uranium to 20 percent is a major step towards purifying it to 90-percent levels needed for a nuclear weapon, although Iran denies intending to do so, saying its activities are peaceful.
Iran has also "placed in position" 2,088 empty centrifuge casings at Fordo, which Iran kept secret until September 2009, and all the piping had been installed, Friday's IAEA report said.
Experts say that once up and running, Fordo, under a mountain near the holy city of Qom, could slash the time needed to convert Iran's stockpile of low-enriched uranium to 90 percent -- if it took the decision to do so.
The centrifuges installed so far at Fordo are older-generation models, however, which experts say could be converted to enrich to 90 percent, but which would do the work more slowly than more state-of-the-art models.
The IAEA said Iran also failed to explain properly what happened to around 20 kilos of uranium metal that the agency says are unaccounted for and which it suspects could have been used in weapons work.
"The discrepancy remains to be clarified," it said.
Diplomats to the Vienna-based IAEA are discussing what action the 35-member IAEA board will take at its next regular meeting from March 5.
Although it could pass a resolution condemning Iran and reporting the Islamic republic to the UN Security Council, which has has already passed four rounds of sanctions calling on Tehran to stop uranium enrichment.
Another resolution would depend on Russia and China who so far have been more lenient on Iran that their Western counterparts.