Secretary of State John Kerry said Sunday that the U.S. and Iran are closer to reaching a final nuclear deal but expressed uncertainty about hitting their 48-hour deadline, saying negotiations “could go either way.”

Kerry made his comments during a break in one-on-one talks in Vienna with Iran Foreign Minister Mohammad Javad Zarif.

Zarif said Saturday that he and Kerry’s teams made significant headway by reaching a tentative agreement on some sanctions now being imposed on Iran for its nuclear program.

However, the deal must be approved by diplomats from the five other world powers involved in the talks, who are returning to Vienna on Sunday.

In addition, major sticking points remain on such issues as inspections of Iranian nuclear-related facilities.

“We have in fact made genuine progress but … we are not yet where we need to be on several of the most difficult issues," said Kerry, who was in his ninth day of negotiations in Vienna. "While I completely agree with … Zarif that we have never been closer, at this point, this negotiation could go either way."

The United States and the five other world powers -- Britain, China, France, Germany and Russia -- have been negotiating with Iran for two years on curtailing that country’s uranium enrichment program toward creating a nuclear weapons.

The sides reached a framework agreement in April, with hopes of reaching a final deal this summer.

Tehran has denied that its enrichment program is for creating a nuclear weapon.

“If hard choices are made this week we can get an agreement,” Kerry also said. “If they are not made they will not. … If there's unwillingness to move on things that are important, of course we will walk away.”

If a deal is clinched by Tuesday, it would set up a decade of restrictions on Iran's nuclear program in exchange for relief from international sanctions.

Tuesday's deadline is the latest that has been set for a comprehensive pact that would replace the interim deal the world powers and Iran reached in November 2013.

The package was extended three times, most recently on June 30.

Critics of the United States’ diplomacy on the issue argue that President Obama's administration has been too conciliatory over the course of the negotiations.

Obama and U.S. officials say that is untrue. But they've also fiercely defended their overtures to Tehran and their willingness to allow the Iranians to maintain significant nuclear infrastructure, on the argument that a diplomatic agreement is preferable to military conflict.

“We want a good agreement, only a good agreement. And we are not going to shave anywhere at the margins in order just to get an agreement,” Kerry said. “This is something that the world will analyze, experts everywhere will look at. There are plenty of people in the nonproliferation community, nuclear experts who will look at this and none of us are going to be content to do something that can't pass scrutiny."

The Associated Press contributed to this report.