Iran and six world powers appeared close to a deal on Monday to give Tehran sanctions relief in exchange for limits on its nuclear program, but Iranian officials said talks could run past their latest midnight deadline and success was not guaranteed.

Diplomats from Britain, China, France, Germany, Russia and the United States wore grim expressions as they met and U.S. Secretary of State John Kerry sat in silence when asked if the deadline might be extended or if he could rule out an extension.

"There shouldn't be any extension," Iranian Foreign Minister Mohammad Javad Zarif was quoted as saying by the semi-official Fars news agency. "But we can continue the talks as long as necessary."

One of his deputies also sounded a cautionary note.

"I cannot promise whether the remaining issues can be resolved tonight or tomorrow night. Some issues still remain unresolved and, until they are solved, we cannot say an agreement has been reached," Iran's Tasnim news agency quoted Deputy Foreign Minister Abbas Araqchi as saying in Vienna.

Diplomats close to the talks said there were contingency plans for an announcement ceremony on Monday if the negotiators sealed an agreement, which would open the door to ending sanctions that have crippled Iran's economy, in exchange for at least a decade of curbs on its nuclear program.

The Western powers in particular suspect Iran may have sought to use its civil nuclear program as a cover to develop a nuclear weapons capability. Iran says the program is solely for peaceful purposes.

The six major powers have given themselves until Monday to reach a deal with Iran. If they fail to get one by midnight, they will need to extend the terms of an interim nuclear deal with Tehran that has already been extended three times in two weeks.

Another option is to walk away, something both the Americans and Iranians have said they are willing to do. They could also suspend the talks for a few weeks or months, though Iran has said it opposes this.

"MAJOR ISSUES"

Kerry has cautioned that "major issues" remain to be resolved, and comments from both senior Republican and Democratic senators on Sunday suggested that any final deal would also face tough scrutiny in the U.S. Congress.

"The parts of the deal are there," said a senior official from one of the six countries. "We still need to put the finishing touches together?. All sides have to decide now. It's time to say 'Yes'."

A senior Iranian official said 99 percent of the issues had been resolved, adding: "With political will, we can finish the work late tonight and announce it tomorrow."

Among the biggest sticking points in the past week has been Iran's insistence that a United Nations Security Council arms embargo and ban on its ballistic missile program dating from 2006 be lifted immediately if an agreement is reached.

Russia, which sells weapons to Iran, has publicly supported Tehran on the issue.

Other problematic issues are access for inspectors to military sites in Iran, explanations from Tehran of past activity that might have been aimed at developing a nuclear weapon, and the overall speed of sanctions relief.

Israeli Prime Minister Benjamin Netanyahu's point-man on the Iranian talks, Energy Minister Yuval Steinitz, told Israel�s Army Radio he was expecting an agreement on Monday or Tuesday. He reiterated Israel's opposition to the deal.

"What is being drafted, even if we managed to slightly improve it over the past year, is a bad agreement, full of loopholes," he said. "If we call it by its true name, they are selling the world's future for a questionable diplomatic achievement in the present."

By Reuters