Treasury Secretary Steven Mnuchin has concerns about Facebook's proposed cryptocurrency and its potential illicit use.

In a press conference Monday, Mnuchin said Facebook's planned digital currency "could be misused by money launderers and terrorist financiers" and that it was a "national security issue."

"Cryptocurrencies such as bitcoin have been exploited to support billions of dollars of illicit activity like cyber crime, tax evasion, extortion, ransomware, illicit drugs and human trafficking," Mnuchin said, adding that he is "not comfortable today" with Facebook's launch.

"They have a lot of work to do," he said.

The press conference comes days after President Donald Trump said in a tweet that he was "not a fan" of cryptocurrencies like bitcoin. He also suggested Facebook, which plans on launching the global cryptocurrency next year, would need a bank charter to do so. Bitcoin dropped sharply on Monday following the president's criticism on Twitter. The world's first and most valuable digital currency fell roughly 10% to a low of $9,872 to start the week.

"The president does have concerns as it relates to bitcoin and cryptocurrencies — those are legitimate concerns that we have been working on for a long period of time," Mnuchin said.

In response to the Treasury secretary's comments, Facebook told CNBC that "they anticipated critical feedback from regulators, central banks, lawmakers around the world." The tech giant also said they announced Libra a year before its anticipated launch date, "so that we could have those conversations."

Facebook's David Marcus, head of Facebook's Calibra digital wallet that will be used to store Libra, is scheduled to testify before the committee on Tuesday. The House Financial Services Committee will hold its own hearing focused on Libra on Wednesday. Marcus responded to questions from the U.S. Senate Banking Committee in a letter last week, saying the company needs governments, central banks and regulators involved to properly launch the digital asset and Facebook "can't do this alone."

WATCH: Facebook responds to Mnuchin's criticism