US President Donald Trump is set to "decertify" the landmark 2015 agreement which curtailed Iran's nuclear programme in return for sanctions relief, leaving lawmakers to decide whether to withdraw completely.
Stoltenberg refused to be drawn on whether he thought the deal was working, but stressed that compliance with its conditions was essential if it was to have any meaning.
"It is not for NATO to make assessments about compliance, that's for nations that are part of the agreement and the IAEA to make that kind of assessment," he told AFP in an interview.
And he reiterated NATO concerns about issues not covered in the deal, in particular Iran's ballistic missile programme.
"The nuclear deal covers the development of nuclear weapons but it doesn't cover missile programmes and we are concerned about the continuous development of missile capabilities of Iran," he said.
Trump has derided the agreement as "the worst deal" and accused Tehran of not living up to the "spirit" of it, but UN inspectors say Iran is meeting the technical requirements of its side of the bargain. International allies, particularly the EU, have lobbied for it to stay, arguing that it is effective.
But last month Iran said it had successfully tested a new medium-range missile with a range of 2,000 kilometres capable of carrying multiple warheads, in defiance of warnings from Washington.
A decision by Trump to decertify the deal would leave it at grave risk, with the US Congress having 60 days to decide whether to re-impose specific sanctions on Tehran that were lifted because of the diplomatic pact.
It would risk unpicking 12 years of careful diplomacy between Iran and six world powers -- Britain, China, France, Germany, Russia and the US -- who crafted the deal.
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