Right up until the last minute, Secretary of State John Kerry thought months of US-Iran negotiations, some held in deep secret in Oman, might fail and he would again leave Geneva empty-handed.

As the top US diplomat prepares to sell the deal on reining in Iran's nuclear program to Congress, a top State Department official revealed Monday that months of back-channel talks between US and Iranian officials had taken place in Oman and other places that have not been made public.

The Omanis were an important conduit for the talks, the official told reporters travelling back from Europe on Kerry's plane, asking not to be named given the sensitivity of the issue.

Washington and Tehran have not had diplomatic ties for three decades, since the 1979 Islamic Revolution led to the subsequent 444-day hostage taking at the US embassy in Tehran, causing relations to be severed in 1980.

And it was a historic moment when President Barack Obama in September telephoned his newly-elected Iranian counterpart Hassan Rouhani as he was leaving New York after the UN annual General Assembly.

But in fact White House and State Department officials, believed to include Deputy Secretary Bill Burns and Jake Sullivan, national security advisor to Vice President Joe Biden, had been in secret contacts with Iranian officials for some time.

It was not immediately clear how many times the two sides met clandestinely, but a US official told the Al-Monitor website that it was "several times" going back to before Rouhani's election in June.

On the Iranian side, the talks involved Deputy Foreign Minister Majid Ravanchi and Deputy Foreign Minister Abbas Araqchi, Al-Monitor said.

But the depth of the contacts stretch back to when Kerry was still a senator, when as chairman of the Senate Foreign Relations Committee he had made an unannounced trip to Oman, the State Department official told reporters on the plane.

The purpose of that trip was to see if Oman was interested in helping to facilitate a dialogue between the US and Iran.

In May, Kerry visited Muscat again as secretary of state, ostensibly to discuss an arms deal with Sultan Qaboos, but it emerges now that it was more likely they talked in depth about the secret negotiations.

Right up until "the very early morning hours" on Sunday, Kerry was unsure whether, despite the months of haggling, a deal would be reached in Geneva or not.

"The last meeting ... with him was pretty much make or break," the State Department official said of a final three-way meeting between Kerry, Iranian Foreign Minister Mohamed Javad Zarif and EU foreign policy chief Catherine Ashton just after midnight in Geneva.

But the official stressed the final deal was not actually struck at that meeting, while declining to say exactly how and when it was sealed.

Although Kerry had felt throughout the day that a deal was possible, he had moments of doubt especially when at one point Zarif looked very anxious and appeared to be under pressure from Tehran, the official said.

Now the US administration is seeking to rally US lawmakers around the interim six-month deal agreed with Iran and the so-called P5+1 group comprising Britain, China, France, Germany, Russia and the United States.

With Congress on recess for the Thanksgiving holidays, Kerry is planning a telephone blitz with his former colleagues in both the Senate and the House.

Following the break he'll be up on Capitol Hill "on day one," the State Department official said, in a bid to dissuade lawmakers from carrying out a threat to impose new sanctions on Iran.

The administration does not believe that simply cranking up sanctions will get Iran to dismantle its suspect nuclear program, the official said, warning such a move would simply drive the Iranians away from any comprehensive deal.

Should the Iranians flout the terms of the six-month interim accord, the $7 billion in sanctions relief awarded in return for freezing parts of their nuclear program would be cut off.

"If they don't do what we need them to do -- bang, back go the sanctions," the State Department official said.