The development of the US reviewing the sanctions relief for Iran comes as Washington also asserted that Iran remains a "leading sponsor" of terrorism.
"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.
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"When the inter-agency review is completed, the administration looks forward to working with Congress on this issue," Tillerson said.
The six powers that negotiated the 2015 nuclear deal — the US, China, Russia, France, Germany and the UK, with the involvement from the European Union — set aside Iran's alleged support for terrorism in order to get a deal guaranteeing that Iran would not be able to build a nuclear weapon for a decade and would remain under the eye of UN weapons inspectors.
The certification of Iran's compliance, which must be sent to Congress every 90 days, is the first issued by the Trump administration.
On February, Trump had described the nuclear deal with Iran as "the worst" agreement ever negotiated, calling the Islamic Republic the number one terrorist state in the world.
Israeli Prime Minister Benjamin Netanyahu had criticised the deal. He said in July of 2015 that "Iran is going to receive a sure path to nuclear weapons."
Former US President Barack Obama had said the deal would make the world safer and more secure. He had said in January of 2016 after the deal was implemented that "Iran will not get its hands on a nuclear bomb."
Iran has defended its nuclear programme as purely civilian.