The head of Iran's Atomic Energy Organization, Ali Akbar Salehi, told lawmakers on Sunday that Iran will restart activities at the Arak heavy water nuclear reactor, the ISNA news agency reported.
ISNA cited a member of parliament who attended the meeting. Heavy water can be employed in reactors to produce plutonium, a fuel used in nuclear warheads.
In May, Iran announced planned measures to breach the nuclear agreement with major world powers following the US withdrawal from deal and Washington's reimposition of tough sanctions.
On July 3, President Hassan Rouhani said Tehran would increase its uranium enrichment levels and start to revive its Arak heavy-water reactor after July 7 if the nations in the nuclear pact did not protect trade with Iran promised under the deal but blocked by the US sanctions.
On Friday, US officials said Iran had test-launched a medium-range ballistic missile inside its borders, defying Trump administration demands that it curtail the weapon program and demonstrating its intent to further push back against US sanctions.
The test came amid heightened tensions between Iran and the West, mainly over the safety of commercial shipping in the Persian Gulf and the Strait of Hormuz.
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A White House spokesman called the test launch an example of Iran "acting out" as a result of intense pressure from US economic sanctions.
"You've seen their economy teetering on the verge of collapse for a while now. And when they're backed into a corner, they're acting out," said spokesman Hogan Gidley, who also said President Donald Trump wants to begin conversations with Iran's leaders.
Iran has responded to stepped-up US economic sanctions with a variety of military moves, and the Shahab-3 missile test launch could be considered another signal from Tehran that it will not back down.
The US officials who confirmed the missile launch spoke on condition of anonymity to discuss sensitive information.
Trump insists that Iran must agree to limits on its ballistic missile program, but Iran thus far has refused.
On Saturday, Iran said the missile tests were part of its defensive needs and were not directed against any country.
"An informed source at the armed forces staff said Iran's missile tests are natural within its defensive needs. This missile capacity is not against any country, and only aims to respond to possible aggression," Iranian news agencies reported.
"Iran does not need the permission of any power in the world for its self-defense," the reports quoted the military source as saying.
Iran has ruled out talks with Washington over its military capabilities, particularly the missile program that it says is defensive. It denies the missiles are capable of being tipped with nuclear warheads and says its nuclear program is peaceful.
Behnam Ben Taleblu, an expert on Iranian defense at the Foundation for Defense of Democracies, said the Shahab-3 is a liquid-fueled, medium-range ballistic missile capable of carrying a nuclear weapon.
"The Shahab-3 is the backbone of Iran's class of medium-range ballistic missiles," he said, adding that Iranian news outlets have previously called it one of the country's "Israel-hitting" missiles.
It is derived from a North Korean missile called the Nodong-A and can fly 1,150 to 2,000 kilometers, or up to 1,242 miles, depending on the variant.
"Iran's continued flight-testing has both political and military applications, functioning as a show of resolve against foreign adversaries and to improve the overall reliability of its missile force, which is the largest in the Middle East," he said. "As Iran continues to escalate in response to the maximum-pressure campaign, Washington should expect more missile launches."
Nations still party to the nuclear deal plan to meet in Vienna on Sunday to see to what extent the agreement can be saved. The European Union said the meeting of officials from China, Russia, Britain, France and Germany will be chaired by the EU.
