Washington's main concern about the rise of crypto currencies is to make sure that they are not used for illicit activity, U.S. Treasury Secretary Steven Mnuchin said on Thursday.

"My number-one focus on crypto currency is that we want to make sure it's not used for illicit activity," he told the World Economic Forum in Davos, Switzerland.

Larry Fink, chief executive of BlackRock Inc, the world's biggest asset manager, told the forum that crypto currencies posed a systemic threat that needed to be dealt with globally.

He said they might be something he considered investing in but the crypto currency industry was "more an index of money laundering".