Tehran says it could be "now or never" for the hopes of saving the 2015 nuclear deal. The United States withdrew from the accord last year before re-imposing sanctions on Iran. Tehran says the move makes the accord "ineffective" and has threatened to break its side of the bargain by exceeding the limits set on low-enriched uranium. The stand-off has led the two sides to the brink of all-out conflict. CGTN's Natalie Carney says the remaining signatories of the accord -- China, Russia, Germany, France and the UK -- are meeting in Vienna today to try to find a way forward.