"It is a message. It makes it clear what we expect from them", a diplomat who asked not to be named told AFP.
The resolution is modeled on a report written by the International Atomic Energy Agency (IAEA) ahead of a meeting of the agency's 35-nation board of governors starting next Monday.
The United States charges that Iran is secretly trying to develop nuclear weapons and should be taken to the UN Security Council for possible sanctions.
But it does not have support for this at the IAEA, especially since the EU's big three have since last October adhered to an agreement they worked out with Iran for cooperation in resolving questions about is nuclear program.
"There are two main points they (the Iranians) have to resolve, as the report makes clear. These are research into sophisticated P2 centrifuges that can make bomb-grade uranium and what is the source of highly enriched uranium contamination which IAEA inspectors have found", a Western diplomat said.
The resolution "acknowledges Iranian cooperation in responding to agency requests for access to locations including workshops" on military sites, according to extracts read to AFP by diplomats.
A US official said the United States "feels very sure the IAEA will take the appropriate firm action" at its upcoming meeting.
But diplomats stressed that a showdown with Iran over it alleged nuclear weapons program would not happen in June.
"We don't think we're at that point in the process but we're nearing the point where Iran will have to fish and cut bait," one diplomat said.
Another diplomat said the resolution oulined by the three European powers was "a response to a report on a stage of the investigation."
"Perhaps (at a board meeting) in September, we'll say that there is no more sense in stressing cooperation," the diplomat said.
The resolution said "that Iran's posponement until mid-April" of a visit by IAEA inspectors "resulted in a delay" that made it all but impossible to resolve the Iran question at the June meeting, according to parts of the text read to AFP by diplomats.
The resolution "deplores" the fact that Iran's "cooperation has not been complete, timely and pro-active."