Federal Reserve Chair Janet Yellen called the digital currency bitcoin a “highly speculative asset” that “doesn’t constitute legal tender.”

Bitcoin, the cryptocurrency, has surged about 17 times in value this year, prompting caution from central bankers around the world.

“Bitcoin at this time plays a very small role in the payment system,” Yellen said Wednesday during a press conference in Washington, appearing to downplay it’s ability to affect wider financial markets. She added that bitcoin is “not a stable store of value.”

New York Fed President William Dudley said last month he would be “pretty skeptical” of bitcoin, adding that it is “not a stable store of value.” Reserve Bank of Australia Governor Philip Lowe said this month that the fascination with cryptocurrencies “feels more like speculative mania than it has to do with their use” as a form of payment.