The gathering of the P5+1 -- Britain, China, France, Russia, the United States, and Germany -- will "see if there are ways on which we can improve on the offer (rejected in June in talks in Moscow) to Iran", one senior Western envoy said Tuesday.
"We want to try to give Iran incentives to meet its obligations, but Iran will also have to take steps as well. We will see what they are willing to do," the diplomat said on condition of anonymity.
"It is looking to see what we can do to help bring Iran back to the negotiating table to negotiate seriously ... There is the pressure side of it, but we also want to see what it is we can do to bring them back to the negotiating table."
In high-level talks in May in Baghdad the P5+1 made an offer to Iran, calling on it to suspend some activities, close its most controversial nuclear facility and ship abroad fissile material.
Many in the international community suspect that Iran's growing civilian nuclear programme masks a covert attempt to develop nuclear weapons, something vehemently denied by Iran.
Iran rejected the proposals in June since the six powers stopped short of offering significant and immediate enough relief from sanctions that have started to hit the Iranian economy hard in recent months.
Diplomatic efforts were then put on hold during campaigning for the US presidential election, won on November 6 by Barack Obama, and diplomats and experts say they expect a new round of talks with Iran early next year or perhaps sooner.
"I think most parties are coming to the table with the realisation that we have to consider what is on offer ? I think we have realised that with what was on offer, and what Iran was prepared to accept, there was no meaningful middle ground," a second Western envoy told AFP.
"We need to find that middle ground ? I think that everyone is coming in with their eyes open, I think the (six powers) are realising that they have to do something more creative."
Wednesday's talks, hosted by EU foreign policy chief Catherine Ashton, are due to start around 9:00 am (0800 GMT) and last several hours, officials said.