US 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, the CNN reported.

"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 US and Iranian officials have said differences remain ahead of a third deadline on Friday.

However, Kerry stressed that despite the tough issues that still have to be resolved, progress continues to be made.

"Let me assure you we would not be here continuing to negotiate just for the sake of negotiating," Kerry said. "We're here because we believe we're making real progress toward a comprehensive deal," Kerry said.