Though Iran has long denied nuclear weapon ambitions, it is "dramatically" accelerating its enrichment of uranium to 60% fissile purity, close to the roughly 90% weapons-grade level, the U.N. nuclear watchdog chief told Reuters in December.

Iran alerted the United Nations on Tuesday to what it described as "reckless and inflammatory statements" by U.S. President Donald Trump threatening the use of force, and warned that "any act of aggression will have severe consequences.

Israeli officials have repeatedly signaled that military action against Iran's nuclear program remains an option. Trump, however, has emphasized diplomacy over mi