WASHINGTON (AFP, REUTERS) - President Donald Trump will not ask Congress to "snap back" sanctions against Iran that would kill a landmark nuclear deal.

"A re-imposition of the sanctions," said Secretary of State Rex Tillerson, "would, in effect, say we're walking away from the deal".

The White House is instead calling for new sanctions to be introduced if Iran passes "specific trigger points".

Separately, Senator Bob Corker, the Republican chairman of the Senate Foreign Relations Committee, said he expects to introduce within the next two weeks legislation that will alter US legislation requiring regular reviews of the nuclear agreement to "address flaws" in the pact.

He insisted that measure, if passed, would not violate terms of the deal reached in 2015.