President Donald Trump is deeply sceptical about the 2015 nuclear deal. He extended sanctions waivers in January but said he would not do so again when they come up for renewal in May unless his concerns are addressed.
Kerry, one of the leading negotiators on the deal, told the Munich Security Conference today that "this is untenable. It is not good diplomacy."
He added: "I believe it is absolutely critical for Europe, for the world, to make sure we hold on to this agreement. Because to go backward, we know what the world looks like without the Iran nuclear agreement. Its not a better place.