WASHINGTON — Treasury Secretary Steven Mnuchin said on Monday that he had “very serious concerns” about cryptocurrencies, including the one being developed by Facebook, the latest indication that Washington is preparing to exert its power over digital currencies.

In a briefing at the White House, Mr. Mnuchin said cryptocurrencies posed a national security threat because they can be used to fund illicit activities. He also said that the Trump administration was “not comfortable” with Facebook’s plans to begin a digital payment system, called Libra.

The warnings come after similar comments by President Trump, who said in a series of Twitter posts last week that he was “not a fan” of cryptocurrencies and that their value was volatile and “based on thin air.” Mr. Trump warned Facebook that it must seek a banking charter and follow all bank regulations if it wants to be in the digital currency business.

The Trump administration has said little about cryptocurrencies in the past two years, despite their growing popularity. Mr. Mnuchin has largely described the new technology for creating, moving and storing money as a potential consumer protection issue rather than a threat to the financial system. Regulators have long focused on how to minimize crime in the cryptocurrency industry, but it has tended to be a wonky issue that has not attracted much public attention.