By Fredrik Dahl and Louis Charbonneau
VIENNA (Reuters) - World powers are pressing Iran to stop stonewalling a U.N. atomic bomb investigation as part of a wider nuclear accord, but look likely to stop short of demanding full disclosure of any secret weapon work by Tehran to avoid killing an historic deal.
Officially, the United States and its Western allies say it is vital that Iran fully cooperate with a U.N. nuclear agency investigation if it wants a diplomatic settlement that would end the sanctions severely hurting its oil-based economy.
The six powers face a delicate balancing act at talks in Vienna, due to end by Monday; Israel and hawkish U.S. lawmakers - wary of any rapprochement with old foe Iran - are likely to pounce on a deal if they believe it is too soft on Tehran.
A senior U.S. official stressed that the powers had not changed their position on Iran's past activities during this week's talks: "We've always said that any agreement must resolve the issue to our satisfaction. That has not changed."
Privately, however, some officials acknowledge that Iran may never be prepared to admit to what they believe it was guilty of: covertly working in the past to develop the ability to build a nuclear-armed missile - something it has always denied.
A senior Western official said the six would try to "be creative" in finding a formula to satisfy those who want Iran to come clean about any atomic bomb research and those who say this is simply unrealistic.
If an eventual accord does not put strong pressure on Iran to increase cooperation with the International Atomic Energy Agency (IAEA) by making it a condition for some sanctions relief, it could hurt the IAEA's credibility, some diplomats say.
INQUIRY IS STUCK
While the global powers - the United States, France, Germany, Russia, China and Britain - want to cut back Iran's uranium enrichment program to lengthen the time it would need to build a bomb, the IAEA has for years has been trying to investigate allegations that Iran actually worked on designing a bomb.
"You don't want to undermine the integrity of the IAEA," said one envoy accredited to the agency.
The IAEA issued a report in 2011 with intelligence information indicating concerted activities until about a decade ago that could be relevant for developing nuclear bombs. It said some of these might be continuing.
IAEA Director General Yukiya Amano this week said Iran had again failed to provide the explanations needed for the IAEA inquiry, which has made scant headway in months.
Iran for its part has said these "possible military dimensions" (PMD) are an issue it will not budge on. "PMD is out of the question. It cannot be discussed," an Iranian official said.
Another Western official said many inside the IAEA and Western governments felt uneasy about compromising on the issue, but added: "I believe the PMD issue is not a deal-breaker, even though it probably should be."
Iran denies ever harboring any nuclear bomb ambitions and its supreme leader, Ayatollah Ali Khamenei, has issued a religious decree against atomic weaponry.
Because of this, experts say, it is virtually impossible for Iranian officials to make any admission of such activity.
Tehran may also be wary of giving its enemies a rationale to attack it out of "self-defense".
As the powers weigh how hard to push, some officials and experts argue that guarantees can be secured that nuclear weapons work has been halted without insisting on what would be an embarrassing Iranian "confession".
(Additional reporting by Parisa Hafezi, John Irish and Warren Strobel; Editing by Kevin Liffey)