The UN's nuclear watchdog is to decide on a draft resolution offering Iran a deadline to show it does not have a nuclear weapons programme.
Critics ask why fuel-rich Iran wants nuclear energy too
Germany, France and the UK want to give Iran until November to prove it is not lying about its nuclear intentions.
Iran insists its nuclear programme is solely for peaceful purposes.
But UK Foreign Secretary Jack Straw has warned Tehran its recent actions have "undermined confidence" in assurances it was not building atomic weapons.
"They cannot turn the issue of confidence on and off like a tap," he said at a meeting of EU foreign ministers in Brussels.
Germany, France and UK have expressed alarm that Tehran has stepped back from a promise it gave last year to stop its uranium enrichment efforts.
They want the IAEA to give Iran until November to dispel suspicions about its nuclear intentions - but IAEA chief, Mohamed ElBaradei, has said there was no time limit for completing the investigation.
On his way into the meeting in Vienna, he said it was an "open process".
Iran says abandoning uranium enrichment is "out of the question" but it is prepared to give assurances that it will not build nuclear weapons.
Iran's Supreme Leader, Ayatollah Ali Khamenei, has meanwhile issued an edict saying Islam forbids the use of nuclear bombs, according to Iranian Foreign Ministry spokesman, Hamid Reza-Asafi.
However, as the text of the edict, or fatwa, has not been published, it is not clear if the Ayatollah has expressly forbidden the building of nuclear weapons, says the BBC's regional analyst, Sadeq Saba.
The draft resolution submitted to the International Atomic Energy Agency (IAEA) gives Iran until November to dispel doubts over its programme, at which stage Mr ElBaradei would decide what further action is needed, if any.
The European trio which prepared the draft has already been involved in delicate negotiations to persuade Iran to abandon its nuclear ambitions.
The United States has led concerns that the Islamic Republic is developing a nuclear industry which could be used to build weapons.
On a visit to Israel on Sunday, US Under Secretary of State John Bolton said the US was "determined that they [Iran] are not going to achieve a nuclear weapons capability".
He said the US sought a peaceful solution, but hinted that all options remained open.
Washington wants the issue to go to the UN Security Council which has the power to impose sanctions.