ISTANBUL — The six world powers gathering here for nuclear talks beginning Friday are finding themselves divided over how best to curb Iran’s ambitions while defusing the possibility of a new military confrontation in the volatile Middle East.

Officials from the six countries that will bargain with Iran have acknowledged in recent days significant differences over what a nuclear accord should look like and under what conditions Iran could be granted partial relief from international sanctions that have put an unprecedented squeeze on the economy there.

While united in insisting on substantial concessions from Iran, the six powers — the United States, Britain, China, France, Germany and Russia — will likely not attempt to craft a joint proposal detailing specific steps Iran must take to assure the world that its nuclear intentions are peaceful, diplomats say.

The lack of such a blueprint could allow greater flexibility in negotiations, though some officials acknowledge that Iran could try to exploit the divisions to gain advantage when the parties meet Saturday here in Turkey’s largest city.

“We really do not have a common view of what’s the real offer to be made to Iran to bring it to serious negotiations,” said Russian deputy foreign minister Sergey Ryabkov, offering an unusually blunt assessment during a visit to Washington this week.

He added that the six countries were united in wanting to avoid what he called a “disastrous” military strike on Iran by Israel, which says a nuclear Iran could pose a mortal threat to the Jewish state. “We are all for a diplomatic solution.”

Other diplomats played down the disputes and insisted that the group is united in the areas that count the most.

“All are in agreement on the core principles,” said a senior U.S. official involved in high-level policy discussions on Iran. The official, who spoke on the condition of anonymity, described bilateral talks leading up to Saturday’s meeting as “transparent” and “among the most collegial that I have seen.”

Still, the fractures among the six negotiating partners could add a further complication to what already was viewed as a long-shot effort to persuade Iran to accept strict limits on its nuclear program. Russia and China have both crafted proposals for resolving the crisis, both of which are expected to call for quickly easing sanctions in return for Iranian concessions. U.S. and European diplomats have insisted on a slower timetable pegged to solid evidence of changes in Iranian behavior.

Iran’s reaction

Whether Iran will agree to any of the proposals is unclear. Iran’s initial assent to attend the talks — the first in 15 months between the Islamic republic and the six-nation bloc known as the P5-plus-one — was followed by weeks of wrangling over dates and venues, as well as conflicting signals over whether Iran was prepared to negotiate seriously.

Iranian President Mahmoud Ahmadinejad has repeatedly insisted that Iran would never give up its right to make enriched uranium, and on Tuesday he boasted that his country could withstand years of financial hardship.