Tehran, May 11 (IANS): The negotiations in Vienna to revitalise the 2015 Iran nuclear deal are progressing, the top diplomat of the European Union (EU) said, as he expressed cautious optimism for an agreement to be reached.

Josep Borrell, the EU's high representative for foreign affairs and security policy, told a press conference on Monday that he hoped there will be "non-stop" talks in Vienna to try to clinch a breakthrough on bringing the United States back to the nuclear deal with Iran, reports Xinhua news agency.

"This remains an extremely delicate and intense diplomatic process. I dare to say that I am optimistic. There is a window of opportunity that will stay open for a couple of weeks until the end of the month, but a lot of work is needed, time is limited," Borrell said, adding that he hoped "that the negotiations will enter in a phase of non-stop" in Vienna.

He said the full implementation of the deal by all parties is the best way to ensure the exclusively peaceful nature of the Iran nuclear program.

"What I can say is that these talks have been constructive and there has been some moderate progress."

Indirect talks between the US and Iran in Vienna have been ongoing on how to revive the nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA).

The JCPOA was reached in 2015 between Iran and the P5+1 -- 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 previous US administration led by Donald Trump unilaterally abandoned the agreement and re-imposed sanctions on Tehran.