Hillary Clinton issued a stern warning on Thursday against the proliferation of fake news, branding it an epidemic with "real world consequences" that must be addressed in order to protect the nation's democracy.

The Democrat lost last month's presidential election to Republican Donald Trump in a shock upset, with several Trump critics arguing that the prominence of fake articles shared on Facebook and other social media may have affected the outcome.

The phenomenon nearly turned deadly this week when a rifle-wielding man entered a pizza restaurant in Washington saying he wanted to investigate a fake news story that wrongly stated the Comet Ping Pong restaurant was a center for child abduction linked to Clinton and a top adviser.

"It's now clear that so-called fake news can have real world consequences," Clinton told current and former US senators on Capitol Hill where she attended a ceremony for outgoing Democratic Senate minority leader Harry Reid.