Bitcoins are seen in this illustration picture taken September 27, 2017. REUTERS/Dado Ruvic/Illustration

LUXEMBOURG (Reuters) - The European Central Bank is concerned about the use of virtual currencies because it is unclear who the issuer is and who guarantees them, ECB Executive Board member Yves Mersch said on Friday.

Bitcoin, the original and still the biggest cryptocurrency, has rallied nearly 75 percent in barely a month.

“We are concerned that this is private currency while we still think currencies have something to do with trust,” Mersch told a conference in Luxembourg.

“The public at large has more trust in public currency rather than in private currency where no one knows who is really the issuer and to what extent it is guaranteed,” he added.