Despite obstacles, nuclear negotiators appear increasingly likely to clinch an historic deal on Iran's nuclear program, western and Iranian officials said.
US officials, including Obama, have long said they see at best a 50-50 chance of getting a deal with Iran.
That remains the official line, but diplomats close to the talks tell Reuters the chances are higher than that as foreign ministers and other negotiators head to Vienna next week for the final stage of a nearly two-year process.
Driving the cautious optimism, they say, is not so much progress made in overcoming sticking points as the intense political pressure on the US and Iranian delegations to reach a deal that would end the 12-year nuclear stand-off between Iran and the West.
"We can't rule out failure, but ... it seems more likely that we will get something. Not by June 30, but perhaps in the days that follow," a senior western official told Reuters, referring to the planned deadline for a deal.
Another western official said, "I think there will be an agreement because the two most important players need it."
The six major powers (the United States, Britain, France, Germany, Russia and China) aim to place constraints on Tehran's nuclear activities for specified periods. In return, they would lift international sanctions that have negatively affected Iran's economy.
A preliminary agreement reached in early April left major differences for negotiators to bridge, including the verification regime to ensure full implementation of the deal and the timetable for lifting sanctions.
***Willingness to Compromise
While acknowledging such issues were far from being resolved and could lead to problems and delays, diplomats said there was a willingness to compromise.
Securing a nuclear deal is politically vital for Obama. For Obama, detente with Iran and another long-term US foe, Cuba, may be the only major foreign policy achievements within reach.
"There will be a deal, Americans need it more than we do. This deal will help both countries," said an Iranian official, who put the chances of a final deal at 70 percent.
One former senior negotiator for one of the six powers told Reuters the chances were as high as 60-40.
Kerry appeared this week to soften Washington's insistence that Iran submit fully to a UN investigation into past nuclear activities that some western countries and their allies claim may have been related to nuclear weapons. Iran denies the allegation, saying the work is totally for peaceful applications.
"We're not fixated on Iran specifically accounting for what they did at one point in time or another," Kerry said on Tuesday.
US officials had previously said Iran has to answer the International Atomic Energy Agency's queries and some sanctions relief would depend on it.
A State Department spokesman said on Wednesday that Kerry's remarks had been misinterpreted and US policy was unchanged.
The major powers recently settled another major sticking point - the so-called "snapback" of UN sanctions in the event of Iranian non-compliance with the deal. Diplomats have declined to discuss details, and Iran has yet to agree.
One wild card in the negotiations, diplomats say, remains France.
It is the most skeptical of the six powers, and the closest to Israel and Saudi Arabia. They blocked agreement on an interim nuclear accord with Tehran in November 2013 before consenting to it weeks later.
French diplomats say they are determined to keep the pressure on for a good deal.
"Our differences with the Americans aren't just semantic. We're insistent on verification and the follow-up to verification," a senior French official said.
