In an opinion piece published by The Washington Post on Monday, Kerry said failure to reach a comprehensive agreement over Iran’s nuclear energy program could not be blamed on the “excessive demands on our part” but on Iran because Tehran has to “back up its words with concrete and verifiable actions.”

 Iran has always emphasized that its nuclear energy program is geared to civilian purposes only. The International Atomic Energy Agency has conducted numerous inspections of Iran’s nuclear facilities but has never found any evidence showing that Iran’s civilian nuclear energy program has been diverted to nuclear weapons production.

 However, Kerry urges Iran to choose between “phased relief” from unilateral sanctions and sanctions that “will tighten.”

 “Now Iran must choose,” writes Kerry in his op-ed, threatening that “international sanctions will tighten and Iran’s isolation will deepen” if Iranians choose to stand by “the positions they have articulated”.

 What Iran has always insisted on is that as a committed signatory to the nuclear Non-Proliferation Treaty (NPT) and a member of the IAEA, it will never give up the right to use nuclear technology for peaceful purposes.

 Iran and the five permanent members of the UN Security Council plus Germany reached an interim deal on November 24, 2013, in the Swiss city of Geneva. The six-month deal, which took effect on January 20 and will end on July 20, can be extended.

 However, Kerry says Iran “must show a genuine willingness” to address the US and its allies’ “concerns” or “the United States and our partners will not consent to an extension.”

 Kerry’s remarks came after Israel sent a high-ranking delegation to the United States to discuss a final deal between Iran and the P5+1.