Former US Secretary of State Hillary Clinton says President Donald Trump’s move to undermine the 2015 Iran nuclear deal has made America look “foolish and small.”

Trump unveiled his new strategy against Tehran on Friday by refusing to certify the agreement, known as the Joint Comprehensive Plan of Action (JCPOA), between Iran and the P5+1 group of countries—the US, the UK, France, Russia and China plus Germany.

Clinton told Fareed Zakaria in a CNN interview on Sunday that the move gave Iran the upper hand, because there was no proof pointing to Tehran's non-compliance.

"If Iran is complying, which all the evidence is, then all of a sudden, instead of working to isolate Iran on every issue, we are giving Iran the spotlight — the aggrieved-party spotlight," she admitted.

"That makes us look foolish and small and plays right into Iranian hands," Clinton continued.

Under the JCPOA, the Islamic Republic agreed to limit certain parts of its peaceful nuclear program in exchange for removal of all nuclear-related sanctions by Washington and its allies in the European Union (EU).

In his speech on Friday, Trump accused Iran of committing "multiple violations of the agreement," a claim repeatedly rebuked by the International Atomic Energy Agency (IAEA), America's EU allies and even officials within his own administration.

Clinton said Trump’s decision was a “major mistake” and part of an impulsive pattern that made Washington look unreliable.

"First of all, it basically says America’s word is not good," she said, arguing that leaving a multinational agreement makes it difficult for the US to negotiate any future deal with other countries.

The former Democratic presidential nominee, who lost last year’s race for the White House to Trump, has been a vocal critic of his new Iran policy.

In a separate interview that was aired on Saturday, she told BBC that the Republican head of state’s reasoning was unclear.

"Basically for political reasons or for personal reasons, it's unclear which he is basically throwing open the door to Iran's nuclear program one more time. I think that is very dangerous," Clinton said.

"There is no evidence that on the nuclear program Iran has cheated in the agreement that the UK and other powers entered into with Iran," she added.