Washington, April 22 : A senior US State Department official said that the second round of the talks in Vienna over the 2015 Iran nuclear deal "made some progress", but important disagreements still existed between Washington and Tehran.

The senior official said on Wednesday that the indirect talks between the US and Iran in Vienna "made some progress" in clarifying respective steps to revive the deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), reports Xinhua news agency.

"But clarification doesn't necessarily mean consensus," said the official.

"There still are disagreements and, in some cases, pretty important ones on our respective views about what is required to -- what is meant by a return to full compliance."

The official said the US had provided Iran with examples of three categories of sanctions, including "sanctions that we believe we would need to lift in order to come back into compliance, and the sanctions that we believe we would not need to lift".

The third category of sanctions was much more complicated with further assessment needed, according to the official, some of which were deliberately imposed by the previous Trump administration to hinder a successor administration from returning to the deal.

The official noted that Washington and Tehran had not discussed the sequencing issue, saying the two sides "are still in the process of describing and detailing the steps that each side is going to have to take".

The official, however, emphasised that Washington had made it clear to Tehran that "a sequence that the US does everything before Iran does nothing is not an acceptable sequence".

"We've made some progress," National Security Adviser Jake Sullivan told CNN earlier in the day, referring to the Vienna talks.

"There is still distance to travel, but ultimately the US is committed to returning to the JCPOA on a compliance-for-compliance basis."

Sullivan also said that the United States would not make any moves on this issue until "we have clarity and confidence that Iran is prepared to put its nuclear program back in the box that it was in during Iran nuclear deal".

Iranian President Hassan Rouhani on Wednesday stressed that the new round of talks about the nuclear deal should aim to map out three steps.

"The first step is the lift of sanctions by the US... As the second step, we want to verify the lift of sanctions thereafter," he said, adding that at the third step, Iran will implement its own commitments under the 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, the UK, 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 Tehran.

Disagreements between US, Iran remain in Vienna n-talks