Iran is preparing a counter-proposal after rejecting the U.S. nuclear deal, prompting President Trump to caution that "time is running out."

Even as the United States and Iran grapple with each other’s nuclear red lines, the simple fact of engagement underscores a pragmatic turn for Tehran regarding President Donald Trump.

Iran's supreme leader Ayatollah Ali Khamenei said Wednesday a US proposal for a nuclear agreement was against the national interest, amid sharp differences over whether Tehran can continue to enrich uranium.