The United States and Iran are set to hold a fifth round of talks over the latter's nuclear program in Rome on May 23, and the stakes could hardly be higher.

While rising U.S.-Iran tensions over Tehran's uranium enrichment jeopardize nuclear talks, three Iranian sources said on Tuesday that the clerical leadership lacks a clear fallback plan if efforts to resolve a decades-long dispute collapse.

Iran met with European powers on Friday to discuss its nuclear negotiations with Washington, while US President Donald Trump issued a new threat unless the Iranians "move quickly" towards a deal. "They know they have to move quickly or something bad is going to happen,