During an interview with Spanish public television TVE, Borrell said "most" countries involved in nuclear talks with Iran agreed with the proposal, but that the United States had not yet responded.

The 2015 agreement between Iran and six world powers -- Britain, China, France, Germany, Russia and the United States -- gave the Islamic republic sanctions relief in exchange for curbs on its nuclear programme.

The deal was designed to prevent Iran from developing a nuclear weapon -- something it has always denied wanting to do.

But in 2018, then US president Donald Trump, a strong critic of the deal, unilaterally pulled out and slapped heavier sanctions on Iran.

Earlier this month, after more than a year of talks coordinated by Borrell and his team, the EU submitted what it called a "final" proposed text -- which has not been made public -- to revive the accord.

"Iran responded by saying 'yes but', that is to say they want some adjustments," Borrell told TVE, without providing further details.

During a press conference Monday in Santander in northern Spain, he said Iran's response had seemed "reasonable" to him and it was therefore submitted to the six world powers involved in the nuclear talks.

"We are waiting for their response, most of them agree, but there is still no answer from the United States... we expect to receive a reply during this week," he added.

Iran has accused the United States of stalling a potential agreement, a charge rejected by Washington.

US State Department spokesman Ned Price said Monday that outstanding questions remained about Iran's requested changes to the EU proposal.

"This is why it has taken us some additional time to review those comments and to determine our response of our own," he said.