To what degree do we credit a single unnamed "U.S. official," talking to Fox News about how he saw Cuban military officers in Syria, especially as it comes just as a Fox commentator falsely claimed he worked for the CIA? As you might guess, Cuba has denied it.
My take is that exactly none of these sources--anonymous official, Fox news, or the Cuban government--is trustworthy. So feel free to ignore the story until there is actual evidence. But rumors like this take on a life of their own, which of course is why they are sometimes intentionally put out there. Because senators will get indignant, which they like doing.
"Press reports now indicate that hundreds of Cuban military personnel are on the grounds in Syria, with the purpose of supporting Russia and Iran in bolstering the Assad regime," the Senate Judiciary Committee chairman wrote in a letter to Obama Friday.
You see how some anonymous dude suddenly became "press reports"? Genius!