update
http://news.bbc.co.uk/1/hi/world/mid...t/5037678.stmb .Iran responded that it would talk to the US only if it was allowed to continue uranium enrichment.
But a White House spokeswoman said it was hoped this was not Iran's last word on the subject.
US President George W Bush warned that if Iran failed to change its mind the US and its Western allies would go to the UN Security Council, where they could seek sanctions.
Analysts say the US move was a major policy change and an attempt to regain the initiative in the Iran nuclear issue.
Tehran says its nuclear activities are aimed at energy production.
The US and Western allies suspect Iran of trying to build a nuclear weapon.
Talks between Iran and European powers have been suspended since Iran resumed uranium enrichment.
Potential punishments
At the meeting in Vienna, the five permanent members of the UN Security Council plus Germany are expected to work on a "carrot and stick" package for Iran.