The United Nations nuclear agency has said it is seriously concerned by Iran's failure to declare aspects of its nuclear programme.
Iran had agreed to fully disclose its past nuclear activity
It said in a report that Iran had not declared designs for a centrifuge used to make bomb-grade material.
Iran had also experimented with polonium, a radioactive substance that can trigger a nuclear blast, it said.
Western diplomats told the BBC the report raised questions about Tehran's readiness to co-operate with the UN.
The Iranian government agreed last year to make a full disclosure of its nuclear activities.
"I hope this will be the last time any aspect of the programme has not been declared to us," said Mohammed ElBaradei, head of the International Atomic Energy Agency (IAEA).
Black market sources
The latest IAEA report, obtained by the BBC, said it was seriously concerned that Iran did not declare designs for the advanced P-2 centrifuge.
"The omission from Iran... to any reference to its possession of the P-2 centrifuge design drawings and associated research... is a matter of serious concern," it said.
The recent revelations about the nuclear black market that supplied Libya's atomic programme have also shed light on Iran, the IAEA said.
Tehran insists its nuclear programme is for energy
"The basic technology is very similar [to that of Libya] and was largely obtained from the same sources," says the report.
But the IAEA welcomed Iran's agreement to suspend enrichment activities and to stop the assembly and testing of centrifuges, saying it would help to build confidence.
The US has accused Iran of pursuing a nuclear weapons programme, but Iran says its nuclear programme is for purely peaceful purposes.
Polonium can be used in conjunction with another metal - beryllium - to ensure that the chain reaction leading to a nuclear explosion is initiated at the correct moment.
Polonium-210 is a radioactive metallic substance that does have a number of industrial uses.
The discovery that Iran has both produced and experimented with the substance has nonetheless caught the attention of nuclear weapons experts, says BBC defence correspondent Jonathan Marcus.
In itself, this does not prove one way or another that Iran has or had a nuclear weapons programme, our correspondent says, but it does raise some worrying questions in the minds of inspectors.
And, according to Western diplomats, it underlines the need for Iran to make a full disclosure of its past nuclear activities.
Libya has agreed to give up all of its nuclear weapons-related activities.
In the process, Western intelligence agencies and the IAEA have been able to lift the veil on the shadowy nuclear export operation run from Pakistan by the scientist Abdul Qadeer Khan.