Reaffirming that he will not unilaterally lift sanctions against Iran, US President Joe Biden said in a television interview broadcast on Sunday the Islamic Republic must first adhere to its nuclear deal commitments despite demands from the country's supreme leader.

The thorny exchange highlighted the diplomatic challenge that lies ahead of Biden as he seeks to revive -- without showing weakness -- a key accord rejected by his predecessor Donald Trump.

Asked in a CBS interview, his first since taking office, whether he would halt sanctions to convince Iran to return to the bargaining table, Biden offered a clear reply: "No."

Biden intended to convey that Iran needs to stop enriching uranium beyond the limits permitted under the Iran nuclear deal, a senior Biden administration official said.

Earlier, an official from Biden administration, referring to the deal formally known as the Joint Comprehensive Plan of Action, said, "They have to stop enriching beyond the limits of the JCPOA,"  Reuters reported.

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"There is nothing changed in the U.S. position. The United States wants Iran to come back into (compliance with) its JCPOA commitments and if does, the United States will do the same."

The landmark deal was reached in 2015 by the United States and other powers (China, Russia, Germany, France and Britain) following long negotiations with Iran aimed at preventing it from developing nuclear weapons.

The deal has been hanging by a thread since Trump's decision to withdraw from it in 2018 and reimpose sanctions on Tehran.

Trump argued that the accord did not sufficiently restrict Iran's nuclear program and he complained of its "destabilizing" activities in the region.

Trump resumed the US sanctions on Tehran that had been lifted through the accord, and he pressed reluctant allies to do the same.

Tehran a year later suspended its compliance with most key nuclear commitments.

The Biden administration has expressed willingness to return to the deal, but insisted that Tehran first resume full compliance.

On January 4, Iran announced it has stepped up its uranium enrichment process to 20% purity, far above the 3.67% level permitted by the deal, but far below the amount required for an atomic bomb.

Biden has promised to return to the agreement -- but only on the condition that Iran first renew its original commitments.

Since Biden's inauguration on January 20, the international community has been eager to learn how he would keep his promise -- complicated by Iran's insistence that the sanctions be lifted first.

The standoff became clearer on Sunday, with Biden digging in his heels and the supreme Iranian leader Ali Khamenei also showing no flexibility.

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"If they want Iran to return to its commitments... the United States must entirely lift the sanctions, in practice and not on paper," the ayatollah said Sunday in a televised speech, AFP reported.

"We will then verify if in fact the sanctions were lifted correctly," he continued, adding that this was "the definitive policy of the Islamic Republic."

The Biden administration has remained intentionally vague on how it plans to move ahead -- so much so that the president completely ignored the topic Thursday in his first speech on foreign policy since taking office.

But Secretary of State Antony Blinken spoke Friday with his German, French and British counterparts to present a united front with the three European signatories to the nuclear deal that have denounced Trump's unilateral withdrawal.

They face a tight deadline: Iran has threatened to bar international inspectors from its nuclear sites on February 21 if the US sanctions have not been lifted.