Washington (CNN) Secretary of State John Kerry made clear Thursday that he was not afraid to walk away from the negotiating table as he addressed looming questions about the Iranian nuclear deal currently in the works.

"We are not going to sit at the negotiating table forever. We also recognize that we shouldn't get up and leave simply because the clock strikes midnight," Kerry said from Vienna, where the talks are taking place.

The deal's deadline has been extended twice, and both U.S. and Iranian officials have said differences remain ahead of a third deadline on Friday.

In an interview with CNN's Christiane Amanpour, E.U. Foreign Policy Chief Federica Mogherini said "the moment of truth will come extremely soon, (in the) next hours I think."

In his comments Thursday, Kerry stressed that despite the tough issues that still have to be resolved, progress continues to be made.

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