Britain, France and Germany have presented a draft resolution to the UN nuclear watchdog agency criticising Iran's attitude towards an inquiry.
International pressure made Iran open up to nuclear inspections
News agencies quote it as saying Iran's co-operation with an investigation into its nuclear activities is not "complete, timely and proactive".
However, the draft acknowledges that Tehran has at least granted inspectors access to its nuclear sites.
Iran categorically denies US claims that it is developing nuclear weapons.
The draft resolution has been circulated informally among delegations representing the 35-nation board of governors of the International Atomic Energy Agency (IAEA).
The board will meet next week and decide whether to adopt the resolution.
Reuters news agency says the draft also "deeply regrets" that Iran has not fully suspended all aspects of its uranium enrichment
programme, as it promised to do under a deal struck last year with the three European states.
Diplomatic sources say that Washington is disappointed that the
resolution is not even more critical of Iran, which the US believes is using its nuclear energy programme as a front
to build atomic weapons.
They said US negotiators
would push for harsher language to be used.
An IAEA report published in April said suspicions still hung over Iran's nuclear programme.
It said the discovery of bomb-grade uranium traces at Iranian facilities, and an Iranian attempt to buy centrifuges to speed up the enrichment of uranium, were a cause for concern.
If the IAEA decides there are reasons to censure Iran, the case may be passed to the UN Security Council.