"These remain an extremely delicate and intense diplomatic process. I dare to say that I am optimistic," Borrell, who serves as coordinator for the 2015 nuclear agreement, said after a meeting of EU foreign ministers.

"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 needs to be done, time is limited."

He added that he hoped "that the negotiations will enter in a phase of non-stop in Vienna".

Indirect negotiations between Washington and Tehran have been going on in the Austrian capital since early April, with the other signatories to the deal -- Britain, China, France, Germany, Russia and the European Union -- acting as intermediaries.

The goal is to find a way back to the accord known by its acronym JCPOA, which former US president Donald Trump walked away from and which his successor Joe Biden wants to revive.

For that to happen, the United States and Iran must agree on the lifting of the sanctions reinstated by Trump and on Tehran's commitment to follow the terms of the deal.

Once Trump walked away from the agreement, the Islamic republic started to abandon the constraints on its production of nuclear material.

Diplomats are hoping to get the US back on board before Iranian presidential elections on June 18.

A fourth round of talks opened in Vienna on Friday, with the leader of Iran's delegation at the talks, Deputy Foreign Minister Abbas Araghchi, hailing a "new energy".

Iran said on Monday it may extend an agreement allowing UN inspectors to monitor some key activities if talks with world powers on its nuclear programme continue "on the right track".