“Traces of uranium, traces of any kind, fissile material are traceable and are very, very hard to get rid of”, Mr Kerry insisted on MSNBC, giving the example of how the Iranians have denied access to its military site of Parchin, just outside Teheran Iran has for years rejected IAEA requests to visit the site and denies ever having worked on developing a nuclear weapon, saying Parchin is a non-nuclear facility.

“On the Sunday before we got the deal, I would say about a week out, I had a very sober and nearly depressing conversation with my counterpart”, Kerry said.

Kerry admitted he was uncertain whether a final deal was possible during 20 months of grueling negotiations.

But almost immediately after the top United States diplomat emerged from almost 90 minutes of fresh talks yesterday with Zarif and European Union foreign policy chief Federica Mogherini, he tweeted that things remained tough.

“But there were some down moments”, he said.

While Iranian officials relished in what they considered diplomatic progress, Kerry, however, expressed via Twitter that the two sides still have some “difficult issues” to resolve.

The day began “with a very serious evaluation” on whether an Iranian agreement with the six world powers on its nuclear agreement was even “doable”, Kerry said.

Kerry hailed the agreement as increasing global harmony and strengthening security.

“We have unprecedented ability to see what they are doing”.

The deal provides Iran with economic sanctions relief in exchange for tighter restrictions on its atomic energy capabilities. “There is no question to me that no one is offering a viable alternative to what we have put forward”.

“The United States is flawless instance of Arrogance”.

“And they will free to go forward with their programs”, Kerry added.

The negotiators missed a Friday morning deadline set by the US Congress for an expedited 30-day review of the deal.

Kerry, a Vietnam War veteran, urged lawmakers on Friday to remember the sacrifices of America’s military while weighing the deal’s details. The practice amounts to “psychological war”, Ali Akbar Velayati, a senior adviser to Khamenei, said Friday, according to the Iranian Students News Agency.

“We have the ability always to bring back our own sanctions and we have multiple other ways of addressing their behavior”, said Kerry.