Diplomatic efforts by Iran and world powers to achieve a potentially historic nuclear deal entered uncharted territory Wednesday as a new round of talks started in Vienna.

After three meetings that Washington says have enabled both sides to “understand each other’s positions,” the negotiators aim this time to start drafting the actual text of an accord.

Success could resolve one of the most intractable geopolitical problems of the 21st century, but failure might plunge the Middle East into conflict and start a regional nuclear arms race.

“If the odds of the talks collapsing are high, the stakes of failure are higher,” Ali Vaez, Iran analyst at the International Crisis Group, told AFP. “Time is of the essence.”

Meanwhile Wednesday, US Defense Secretary Chuck Hagel promised American allies in the Persian Gulf that negotiations to contain Iran’s nuclear program will not weaken their security.

In remarks opening a conference with his Gulf counterparts, Hagel said Washington is hopeful of progress this week in the Iran deal-drafting talks in Vienna.

“As negotiations progress, I want to assure you of two things,” Hagel told the Gulf Cooperation Council. “First, these negotiations will under no circumstances trade away regional security for concessions on Iran’s nuclear program.”

The Pentagon chief continued, “Second, while our strong preference is for a diplomatic solution, the United States will remain postured and prepared to ensure that Iran does not acquire a nuclear weapon – and that Iran abides by the terms of any potential agreement.”

Even if Tehran backs out of the nuclear negotiations, Hagel said, “the United States remains committed to our Gulf partners’ security.” He said there are about 35,000 US troops in the Gulf region.

On Monday, US President Barack Obama’s national security adviser, Susan Rice, vowed to keep Iran from obtaining a nuclear weapon and said the United States would work to revive Israeli-Palestinian peace talks.

In her remarks at an Israeli embassy event on Monday marking Israel’s independence, Rice said she discussed Iran last week in meetings in Israel with Israeli Prime Minister Benjamin Netanyahu, and reiterated the US position “that Iran must not get a nuclear weapon.”

A US team led by Wendy Sherman, an undersecretary of state, is in Vienna this week for the latest round of nuclear talks between Iran and the P5 +1, the major powers including the United States, Russia, China, Britain, France and Germany.

“As the United States and our P5+1 partners engage in negotiations with Iran on a long-term, comprehensive agreement that resolves the world’s longstanding concerns about Iran’s nuclear program, we all have a responsibility to give diplomacy a chance to succeed,” Rice said at the embassy event.

The five permanent members of the UN Security Council plus Germany want Iran to reduce in scope its nuclear program so as to make any dash to build an atomic bomb virtually impossible.

In return, the Islamic republic, which says its nuclear activities are purely peaceful, wants the lifting of all UN and Western sanctions, which have hit its economy hard.

Arriving in Vienna Tuesday, both Iran and the United States sought to dampen expectations that a deal was within easy reach, with Zarif saying a “lot of effort” was still required.

A senior US official said the talks would be “very, very difficult” and that there were still “significant gaps” between the two sides and a “range of complicated issues” to tackle.

“We do not know if Iran will be able to make the tough decisions they must to assure the world that they will not obtain a nuclear weapon and that their program is for entirely peaceful purposes,” the official said.

She added that optimism raised in some quarters has “gotten way out of control.”

The parties aim to build on an interim deal struck in Geneva in November under which Iran froze certain activities for six months in return for minor sanctions relief. This expires on July 20.

One major issue, the Arak reactor, appears to have been resolved, with Iran indicating the design could be modified to ease concerns that it could produce weapons-grade plutonium.

But others, most notably uranium enrichment and the sequence of sanctions relief “could be harder to bridge,” Kelsey Davenport from the Arms Control Association told AFP.

Iran already has enough of low-enriched material for several bombs if it decided to “break out” and use its 20,000 so-called centrifuges to enrich this stockpile to weapons-grade.

The powers may therefore want Iran to slash the number of centrifuges, or to cap output per machine, but this may be a hard sell for hardliners in Iran, where enrichment is a source of great national pride.