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By Andrea Mitchell and Abigail Williams

VIENNA — U.S. Secretary of State John Kerry and his Iranian counterpart were in nonstop negotiations Sunday with just 48-hours to go before their latest deadline for a nuclear deal — negotiations he acknowledged "could go either way."

Kerry and Iran's Foreign Minister Mohammad Javad Zarif sat down at around 10:30 a.m. (4:30 a.m. ET) local time in Vienna for talks, taking just a short 30-minute break before resuming meetings in the early afternoon.

Kerry addressed the press during a brief news conference at about 5 p.m. (11 a.m. ET) Sunday before several more foreign ministers were expected to join the talks.

"We have in fact made genuine progress (but) we are not yet where we need to be on several of the most difficult issues," Kerry said. "While I completely agree with Foreign Minister Zarif that we have never been closer — at this point, this negotiation could go either way."

The pressure is mounting: Iran and six world powers have set a Tuesday deadline for a comprehensive deal on Tehran's nuclear program. The U.S. — along with Britain, China, France, Germany and Russia — want curbs on Iran's nuclear program in exchange for sanctions relief.

Their technical staffs reached a tentative agreement Saturday on how to lift U.S. and E.U. sanctions, but that still has to be accepted by the European ministers and China when they return to the talks later Sunday. But there are other major sticking points, including on separate United Nations sanctions.

Kerry said the deadline could be met "if hard choices get made in the next couple of days." And he said the Obama administration is all too aware that the deal will be heavily scrutinized.

"This is something that the world will analyze,” Kerry said. "We want a good agreement and only a good agreement."

Diplomats have yet to determine what to do if Iran does not meet its obligations under any deal; the U.S. wants an automatic "snap back" mechanism for the U.N. sanctions that won't require a Security Council vote.

Gaps also remain on how to insure nuclear inspectors' access to sensitive sites in Iran, and how and when Tehran will be allowed to resume advanced nuclear research.

Officials said Sunday that a deal could be reached by Monday night at the earliest.

If a deal is reached, Kerry must issue a report in writing to the Senate's Foreign Relations committee by Thursday.