With just 48 hours to meet the deadline for an Iran nuclear deal, U.S. Secretary of State John Kerry announced Sunday that negotiations at this point "could go either way" and that "we have difficult issues still to resolve."

"We are not yet where we need to be on several of the most difficult issues," Kerry said. "And the truth is that while I completely agree with [his Iranian counterpart] Foreign Minister Zarif that we have never been closer — at this point, this negotiation could go either way."

Kerry sat down with Iran's Foreign Minister Mohammad Javad Zarif Sunday morning in Vienna for talks, taking only a 30-minute break before resuming meetings in the early afternoon.

"If hard choices get made in the next couple of days, and made quickly, we could get an agreement this week," said Kerry. "But if they are not made, we will not."

"We are not going to be negotiating in the press," added Kerry. "We're aiming to try to finish this in the time frame we've set out."

The two leaders are under pressure to meet a Tuesday deadline set by Britain, China, France, Germany and the United States. Iran and the United States have been at odds over sanctions relief for Iran, and whether Iran will allow international inspectors to verify that its nuclear program is used only for peaceful purposes. Should a deal be reached by the deadline, Kerry will need to issue a report to the Senate Foreign Relations committee by Thursday

Though the countries reached a tentative agreement Saturday to lift U.S. and E.U. sanctions, Kerry sounded harsher than he has previously on the prospect of whether a deal will be reached.

"If we don't get a deal, if there's absolute intransigence… President Obama has always said we will be prepared to walk away," said Kerry. "I've said from the moment I became involved in this: We want a good agreement, only a good agreement. We're not going to shave — anywhere — at the margins in order just to get an agreement."

"None of us are going be content to do something that can't pass scrutiny," said Kerry, mentioning all the experts who will review a deal if it is reached. "Everybody is negotiating hard, that's what makes this difficult."