The original sources seem to be Israeli and they reported it was a conversation between two officials about treatments. Later there was a claim that was attributed Assad’s brother These were reported here in FR’s during the past two weeks.
This report claims a US source
Since the incident took place a week ago today, those reports may have been related to the earlier incidents.
This story does not claim a US source, It claims a US official "overheard" the conversations. It implies the US intercepted the conversations, but they could have heard the israeli tapes.