Secretary of State Antony Blinken said on Thursday that US intelligence agencies have yet to determine the cause of mysterious symptoms affecting US diplomats around the world, nicknamed “Havana Syndrome”.

       Mr Blinken made the comments during an interview with MSNBC after it was reported that more Americans in Geneva, Switzerland, and Paris, France are experiencing the symptoms, resulting in at least one returning to the US.

       The unexplained phenomenon has now spread to roughly 200 people, and is no longer clustered in Havana, which drew the suspicions of US lawmakers and media figures.

       “To date, we don’t know exactly what’s happened and we don’t know exactly who is responsible,” Mr Blinken told MSNBC on Thursday.

       He went on to say that the admission was not an indication that the federal government was not taking the symptoms seriously, and said that his department was working “overtime” to find answers.

       Recommended Democrats push rival bill sanctioning Russia over Ukraine Russia, US hold working dinner to open Geneva talks Prospects dim as US, Russia prepare to meet over Ukraine

       "There is no doubt in my mind that people have been directly and powerfully affected," he said.

       "We are working overtime across the entire government to get to the bottom of what happened, who's responsible. And in the meantime to make sure that we're caring for anyone who's been affected and to protect all of our people to the best of our ability,” added the secretary.

       The comments come after questions have been building for months about the strange, unexplained illnesses and a host of theories have been suggested ranging from the supposed existence of microwave-based weaponry to more germane suggestions.

       More follows...