The UN atomic watchdog hit back on Monday at US criticism of the Iran nuclear deal, insisting its inspections there are the world's toughest and that Tehran is sticking to the accord.

"The nuclear-related commitments undertaken by Iran under [the 2015 deal] are being implemented," International Atomic Energy Agency head Yukiya Amano told reporters.

"The verification regime in Iran is the most robust regime … currently existing. We have increased the inspection days in Iran, we have increased inspector numbers ... and the number of images has increased," he said in Vienna.

"From a verification point of view, it is a clear and significant gain."

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US President Donald Trump has called the agreement between Iran and six major powers reducing its nuclear activities in exchange for sanctions relief "the worst deal ever negotiated".

UN Ambassador Nikki Haley made a case last week for how the United States could back away from the landmark agreement.

"You can't put lipstick on a pig," Haley said. "We have to look at the reality that this deal is flawed."

Trump has to certify in mid-October whether he believes Iran is abiding by the nuclear deal and that sticking to it is vital to US national security interests.

If, as seems increasingly likely, Trump decides not to give the green light, Congress will then have 60 days to debate whether to re-impose sanctions on Iran.

Iran's President Hassan Rouhani said last month the top foreign policy priority for his new government was to protect the nuclear deal from being torn up by the United States.

"The most important job of our foreign minister is first to stand behind the JCPOA and not to allow the US and other enemies to succeed," Rouhani told parliament, using the technical name for the 2015 agreement.