Wrapped up three-days of talks with his Iranian counterpart Mohammad Javad Zarif in the Swiss lake city of Montreux, Kerry said, "Now, we still don't know whether we will get there, and it is certainly possible that we won't. It may be that Iran simply can't say yes to the type of deal that the international community requires."
Kerry said the talks would resume them on March 15.
"From the beginning, these negotiations have been tough and intense, and they remain so," he said said.
Final deal to curb Tehran's nuclear programme would have to withstand global scrutiny, he said.
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"No one has presented a more viable, lasting alternative for how you actually prevent Iran from getting a nuclear weapon. So folks, simply demanding that Iran capitulate is not a plan," he said, apparently referring to Netanyahu's comments in the US Congress that Washington was negotiating a bad deal with Iran that could spark a "nuclear nightmare.
The sticky points in the negotiation from the American side has been the breakout time - the time needed for Iran to assemble a weapon - to be extended to at least a year in order to provide for a reaction time.
Iran, on its part, wants a complete relief from international sanctions which is crippling its economy.
"Sanctions and agreement don't go together. If they want an agreement, sanctions must go," Zarif had said yesterday.
In an address to the US Congress yesterday, Netanyahu said that the proposed nuclear agreement would not stop Tehran from getting nuclear weapons but would rather pave its way to more of them which would imperil the Jewish state. Netanyahu called for more sanctions against Iran.
Debunking the argument that sanctions are an effective deterrent against a nuclear Iran, Kerry said, "We also know that the international sanctions, which many want to simply hang their hats on - they may have gotten Iran to the table, but to date they haven't stopped Iran from advancing its nuclear program.
"Clearly, increased breakout time makes any nation in the vicinity or any nation of concern safer," he added.
With the lapse of two deadlines, the P5+1 and Iran have until March 31 to agree on a framework accord on Iran's nuclear facilities.