Why are tensions so high as Iran and US prepare to discuss nuclear issues?

An Iranian demonstrator holds up an anti-US placard and a poster of the Supreme Leader Ayatollah Ali Khamenei (Vahid Salemi/AP)

Iran and the United States will hold talks in the sultanate of Oman on Saturday in an attempt to jump-start negotiations over Tehran’s rapidly advancing nuclear programme.

Even before the talks, however, there was a dispute over just how the negotiations would go.