"Iran remains a leading state sponsor of terror, through many platforms and methods", Tillerson wrote in a letter to the House of Representatives Speaker Paul Ryan, in which he certified that Iran has so far complied with the provisions of the nuclear agreement.
The nuclear deal was sealed in Vienna in July 2015 after 18 months of negotiations led by former secretary of state John Kerry and diplomats from the other four permanent members of the UN Security Council-Britain, China, France and Russia-and Germany.
He said the deal, brokered by former President Barack Obama's administration along with other world powers, represented the "same failed approach" the US has taken to North Korea.
In the first reaction to Tillerson's remarks from a senior Iranian official, Zarif tweeted that the United States should "fulfill its own commitments". On the one hand, Trump wants to show he's being tougher than Obama toward Iran, but on the other hand, he's not yet ready to rip up the deal. Obama and others argued it was narrowly tailored to take the most risky prospect - a nuclear-armed Iran - off the table.
In the deal, countries were to lift trade sanctions against Iran, in exchange for the country halting development on its nuclear programme. With some of those critics now in office, Tillerson's comments Wednesday marked the first time that position has been echoed by the USA government. However, the terms of the nuclear deal give the administration leeway in terms of enforcement, Foreign Policy reported Monday: "The Trump White House is mulling taking a much more forceful stance on enforcing the deal to the letter".More news: JaVale McGee disarms critics as Warriors squash Portland in Game 2
"Iran has not lived up to the spirit of the agreement and they have to do that", Trump said, adding that the administration was "analyzing it very, very carefully" and would have more to say about it soon.
But Tillerson is leaving open the possibility that the Trump administration will uphold it anyway.
Opponents of the deal in the U.S. objected, claiming it did not allow for the kind of inspections of Iran's atomic sites that would guarantee the country's nuclear activities remain peaceful.
"An unchecked Iran has the potential to travel the same path as North Korea and to take the world along with it", Mr Tillerson said. Founded in 1846, AP has covered all the major news events of the past 165 years, providing high-quality, informed reporting of everything from wars and elections to championship games and royal weddings.
Iran has been on the list since 1984, and "continued its terrorist-related activity in 2015, including support for Hizballah, Palestinian terrorist groups in Gaza, and various groups in Iraq and throughout the Middle East".