But the window of opportunity for a negotiated solution is narrowing because of growing alarm over Tehran's nuclear course in Israel, which has threatened to bomb Iranian installations.
Any Israeli strike, which would spark more fighting in the Middle East, is unlikely before the country's January 22 election, experts say, giving the six powers some room for maneuver.
"There certainly is a window to do a deal, but that window is closing, and closing fast. Ultimately it depends on the Iranians meeting their international obligations," said Ariel Ratner, former Obama administration political appointee on Middle East issues at the State Department.
In hopes of a breakthrough, and despite deep skepticism a deal with Tehran can be reached, the powers - Britain, China, France, Germany, Russia and the United States - are revising their strategy after three inconclusive rounds of negotiations this year.
Their plan could be presented to Iran in talks, convened by EU foreign policy chief Catherine Ashton, which diplomats say may take place in the coming months in Istanbul.
"The idea of the (Wednesday) meeting is to coordinate on what kind of offer we are going to go forward with to Iran," said one senior Western official, familiar with the planning.
Iran denies international accusations it is seeking nuclear weapons and has so far refused to meet demands to scale back its atomic activity, insisting on immediate relief from sanctions.
Western powers have rejected that in previous rounds, instead offering limited incentives focused on technology cooperation. They have also ramped up punitive measures intended to get Iran, one of the world's biggest oil producers, back to the table for meaningful talks.
Sanctions pressure increased last month when European Union governments stepped up restrictions on trading with Tehran and banned imports of Iranian gas, to complement this year's crippling embargo on Iran's crude oil.
LITTLE PROGRESS
There are scant signs sanctions are pushing Iran towards a deal. A U.N. report showed this month Iran is set to sharply expand uranium enrichment in its underground Fordow plant.
Diplomats say one option being considered by the six powers is to ask Iran for more concessions, while offering more substantial sanctions relief.
"With more space in Washington and more pressure in Tehran, there might just be room for a deal," said Jon Wolfsthal, a former adviser to U.S. Vice President Joe Biden. "Sanctions relief has to be on the table at some point, but Iran has to be able to get to 'yes'."
In the earlier meetings this year, the powers called on Iran to stop producing higher-grade enriched uranium, shut down the Fordow facility and ship out its stockpile.
Iran rejected the proposal, described by Western officials as an initial step to build confidence, and demanded recognition of its "right" to refine uranium, activity which can have both civilian and military purposes, as well as lifting sanctions.
Any new deal would have to be carefully choreographed to entice Iran to make concessions while taking into account deep-seated reluctance among Western governments to ease sanctions.
"The key is sequencing the significant steps," said Cliff Kupchan, a Middle East analyst at consultancy Eurasia Group. "My guess is that's what they (the six powers) will discuss."
