Newly appointed U.S. Treasury Secretary Janet Yellen called Bitcoin a "highly speculative asset" and pointed to the asset's high volatility in an interview with CNBC on Thursday.

When asked whether cryptocurrencies like Bitcoin need a special regulatory environment, Yellen said that authorities are focusing on protecting investors and ensuring that cryptocurrencies are not used in criminal activities.

"I think it is important to ensure that it is not used as a vehicle for illicit transactions and that there is investor protection," said Yellen.

After that, Yellen went on to talk more about cryptocurrencies, adding that, for the agencies that oversee the digital asset space, it is “certainly important” to ensure that they follow their regulatory responsibilities.

The United States Treasury Secretary has been a vocal critic of Bitcoin in the past and, during her Senate confirmation hearing in January, she said cryptocurrencies were a “particular concern” when it comes to facilitating criminal activity and sponsoring terrorism . Later, however, Yellen recognized its "potential" for improving the financial system.