The E3 letter follows months of diplomatic stalemate, particularly after a 12-day war between Iran and Israel in June.

Iran has conceded it will limit its nuclear programme and reduce uranium enrichment levels if the United States lifts sanctions.

The UK, France and Germany have told the UN they are ready to reimpose sanctions on Iran over its nuclear programme if it fails to resume talks by the end of August. The three countries, known as the E3, said they were prepared to trigger a "snapback" mechanism - meaning previous sanctions would be reinstated - unless Iran resumes negotiations.