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.

“They — the West — intend to impose an embargo on our oil,” Ahmadinejad said in a defiant speech carried live on state TV. “We have as much hard currency as we need, and the country will manage well, even if we don’t sell a single barrel of oil for two or three years.”

At the same time, other Iranian officials have signaled a willingness to bargain. Borrowing from the West’s diplomacy playbook, the head of the Atomic Energy Organization of Iran publicly floated a possible compromise this week, suggesting that his government might agree to stop making 20 percent enriched uranium, a more purified form of nuclear fuel that can be quickly converted to weapons-grade uranium.

“Our systems are capable of making this change,” the official, Fereydoun Abbasi, told the Iranian Students’ News Agency.

Iran says its nuclear program is only for peaceful purposes, such as medical research and electricity generation. But the United States and other nations have repeatedly accused Iran of seeking at least the capability to make a nuclear warhead.

Time running out for talks

The Obama administration and its European allies are hoping that Iran will agree to significant concessions in the face of unprecedented economic pressure, including the threat of an oil embargo set to begin in July and severe new sanctions against Iran’s central bank.

Senior U.S. officials have said publicly that the window for diplomacy is closing, suggesting that a failure by Iran to scale back its nuclear program voluntarily could result in a military strike by Israel. U.S. officials have warned that such a conflict could spread through the region as Iran and its proxies hit back against Israel and its allies.

“We believe there is still time for diplomacy, but it is urgent that the Iranians come to the table,” Secretary of State Hillary Rodham Clinton said Wednesday at a Group of Eight plenary session in Washington.

The talks in Istanbul will begin Friday with an informal gathering of the P5-plus-one negotiators, followed by negotiations with the Iranian delegation on Saturday. Diplomats have been playing down expectations for a breakthrough, but some point to the possibility of a second round of talks in the coming weeks if progress is being made.

“The reality is, we have to wait and see what the Iranians have to say, and whether they’re truly serious this time,” said a Western diplomat whose government is participating in the talks and who spoke on the condition of anonymity.

The session on Friday will provide the six-nation bloc an opportunity to narrow differences in views about Iran, even if a common position is unlikely. While leaders of the six governments publicly stress areas of agreement, diplomats acknowledge sharply differing views on key issues, including sanctions — specifically, whether existing ones are effective and how quickly such punitive measures could be withdrawn in return for Iranian concessions.

Difficulties with diplomacy

Ryabkov, the Russian diplomat, said continued economic pressure has only deepened Iranian defiance.

“We have never seen any movement on the Iranian part because of pressure,” he told a gathering of the European Institute, a Washington policy forum. “We only saw more stubbornness.”

Ryabkov disclosed that his country had reached out to Iran separately to help its government work through details of a possible compromise. What he termed a “drafting phase” ultimately failed because Iranian officials insisted that any agreement include an immediate lifting of economic sanctions. Western countries say sanctions would have to be eased gradually, after U.N. nuclear inspectors verify that Iran has followed through with promises to scale back its nuclear activities.

To gain any relief from sanctions, diplomats say, Iran would have to come clean about past nuclear weapons research and agree to halt production of the more highly enriched, 20 percent nuclear fuel.

Some diplomats, echoing a view voiced by senior Israeli officials and conservative U.S. politicians, say Iran should not be allowed to enrich uranium at all.

Yet all sides will face pressure to show progress, or least sufficient momentum to warrant a second round.

“Both political systems need a deal,” said Clifford Kupchan, a former State Department official and a now an Iran expert at the Eurasia Group, a Washington consulting firm. “Washington needs concessions to restrain Israel, and Iran needs Western concessions to keep its economy from tanking further. That’s enough, at least, to make it probable that this process will keep going for a while.”