According to Rouhani, the third step will be taken on September 6 after talks between Iran and the Group 4+1 (France, Russia, Britain, China and Germany) did not bear "the desired result."
"We took the first step in reducing our commitments, and gave the Group 4+1 a two-month deadline. Then we went ahead with the second phase, giving them another two-month deadline. In the four-month period, we held negotiations with the Group 4+1, including the European Union and the three European countries in particular," Rouhani said.
The Iranian president made the announcement in a press conference on Wednesday night, following a trilateral meeting with Iran's Judiciary Chief Seyed Ebrahim Raisi and Parliament Speaker Ali Larijani, Press TV reported.
Since May, Iran has been suspending some of its commitments under the nuclear deal, officially known as the Joint Comprehensive Plan of Action (JCPOA), which was clinched between Tehran and world powers in 2015. The country has already rowed back on its nuclear commitments twice in compliance with articles 26 and 36 of the JCPOA.
He added that Iran did not achieve the desired result from talks with the remaining JCPOA parties, and will thus take the third step on Friday.
In the third phase, Rouhani said, "the Atomic Energy Organization of Iran (AEOI) will be obliged to immediately start research and development on whatever technical needs the country has, and set aside all R&D commitments stipulated in the JCPOA."
The president said the AEOI is expected to expand its "R&D on various new centrifuges and whatever the country may need for uranium enrichment" at a fast pace.
"A major part of negotiations with the Group 5+1 (Russia, China, the US, the UK, France, and Germany) was focused on the R&D timing... All the R&D timings in the JCPOA to which we committed ourselves will be fully lifted as of Friday," Rouhani said.
"We will carry out whatever we need technically... under the supervision of the International Atomic Energy Agency (IAEA) and within a peaceful framework," he went on to say.
Rouhani at the same time stressed that the cuts are reversible if other parties to the JCPOA fulfill their obligations.
"We still give another two-month time to the Group 4+1, during which we may return to our JCPOA commitments in case we reach an agreement," he added.
Earlier in the day, Rouhani had stated that the third phase will begin as it was "unlikely that we will reach a final agreement with Europe today or tomorrow."
The president described the third phase of Iran’s commitment reductions as “highly important in nature,” emphasizing that this would the “most significant step that we take” and would “have extraordinary effects.”
This phase, Rouhani added, will "accelerate" the activities of the AEOI.