TEHRAN - Head of the Atomic Energy Organization of Iran, Ali Akbar Salehi, on Sunday warned the United States against any move which undermines the 2015 landmark nuclear deal between the Islamic republic and the world powers.

In case the United States decides to walk away from the deal, Iran would be able to resume 20 percent enriched uranium in four days, Salehi told Iran's Parliament news agency.

Returning to the production of 20 percent enriched uranium "bears messages for the United States," he said without elaboration.

Iran has long considered the appropriate responses to the possible quit of the nuclear deal by the United States, he said, adding that "we hope the Americans do not drop the deal ... since it would be a scientific challenge for them."

On Thursday, Salehi warned the United States not to make any more move against the 2015 Iranian nuclear deal.

"We are seriously looking to preserve our national interests and sovereignty, but if the United States withdraws and Europe and other great powers retreat from this agreement, we will definitely do something different," said Salehi.

Salehi described U.S. President Donald Trump as a businessman who tries to advance his international agendas with unpredictable decisions.

Trump has repeatedly demanded the elimination of sunset clauses for some of the restrictions the United States places on Iran from the nuclear deal, stronger inspection rules, and limitation to Iran's development of long-range missiles.

But Iran says it will neither tolerate anything beyond its commitment to the nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), nor accept changes to the agreement.

Iran also said it would be "humiliating" for the European countries to follow the U.S. policy on the nuclear deal, according to Press TV.

Iran's atomic chief said the main responsibility of his organization is to make preparations to deal with this scenario, adding that the organization has already got ready in this regard.

The U.S. ambassador to the United Nations, Nikki Haley, said on Thursday that Washington would probably get out of the Iran nuclear deal and reimpose sanctions on the Islamic Republic.

She accused Iran of supporting terrorism and violating the terms of the JCPOA. Europeans were closing their eyes to this, she added, saying that the United States would not do so.

Iran has categorically dismissed the allegations.

On Saturday, a senior Iranian lawmaker said that his country will definitely walk away from the 2015 nuclear deal if the United States reimposes sanctions on Iran.

"The most important objective of the JCPOA was the lifting of sanctions" against Iran, said Alaeddin Boroujerdi, Head of Iran's Majlis, or parliament, National Security and Foreign Policy Commission.

"If sanctions are reimposed on Iran, we will definitely not remain in the JCPOA," Boroujerdi was quoted by Press TV as saying.

Iran said it will not take any measure beyond its commitment to the JCPOA, nor will it accept changes to this agreement now or any time in the future.

Salehi said that domestic experts have managed to renovate Tehran Research Reactor (TRR), a 5 megawatt-thermal pool-type light water research reactor, Tasnim news agency reported on Sunday.

He said 40-year-old TRR needed complete renovation and efforts were made for it.

In addition to the renovation of the reactor, a nuclear fuel test loop has been manufactured and installed for the first time to test the resilience of the nuclear fuels due to be used in power plants, he added.