Washington, May 8 (IANS): US President Joe Biden on Friday raised concerns about Iran's seriousness in Vienna talks to revive the 2015 nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA).

Asked at the White House if he thought Iran was serious about the ongoing negotiation in Vienna, Biden replied, "Yes. But how serious and what they're prepared to do is a different story", the Xinhua news agency reported.

White House Press Secretary Jen Psaki later in a daily press briefing recognized that some progress had been made in past weeks' talks, adding it is a good sign that the negotiations are continuing.

Meanwhile, she reiterated that the United States "would be prepared to lift the sanctions necessary for our JCPOA compliance only if Iran were prepared to return its nuclear program to its JCPOA status."

"We will not get into a situation where the United States does more than is required by the JCPOA agreement in terms of sanction relief and Iran does less," she noted.

The fourth round of talks over a potential US return to the Iran nuclear deal began in Vienna on Friday.

A senior State Department official said on Thursday that the United States and Iran could achieve a mutual return to compliance with the Iran nuclear deal in the coming weeks if Tehran makes a "political decision."

Iranian President Hassan Rouhani said on Wednesday that the United States knows that it has to return to law and assume its obligations pertaining to the nuclear deal.

The JCPOA was reached in 2015 between Iran and the P5+1 (the five permanent members of the United Nations Security Council -- the US, Britain, Russia, France, China, plus Germany) together with the EU. Tehran agreed to roll back parts of its nuclear weapons program in exchange for decreased economic sanctions.

Iran gradually stopped implementing parts of its commitments in May 2019, one year after the administration of former US President Donald Trump unilaterally abandoned the agreement and re-imposed sanctions on Iran.