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Such investments should be registered in the same way as securities sales, according to the Securities and Exchange Commission (SEC). It follows a rise in the number of Initial Coin Offerings, or ICOs, through which crypto-currency start-ups have sought quick financial backing. However, there are concerns that ICOs are creating an investment bubble. More than $1bn has been raised so far this year in such transactions but there are questions over how investors are to be protected should a venture backed with digital coins fail. Now the SEC has said that issuers of "distributed ledger or blockchain technology-based securities" must register offers and sales. "The innovative technology behind these virtual transactions does not exempt securities offerings and trading platforms from the regulatory framework designed to protect investors and the integrity of the markets," said Stephanie Avakian, the SEC enforcement division's co-director. 'Large valuations' It was sometimes difficult to see how a firm had justified a large evaluation for its ICO, said Eitan Jankelewitz, a technology specialist and partner at law firm Sheridans. "If you consider the size of the investment compared to the type of investment that a professional would make, they look disproportionally large," he told the BBC. He also pointed out that regulators don't need to rely on legislation specifically dealing with crypto-currencies to enforce rules on securities and investments. "Those sorts of regulations tend to be more principles-based," he explained. "If a new technology comes along that allows a new activity to be undertaken, then the law would still apply." The SEC made its comments on ICOs in a report on a crypto-currency investment fund known as the DAO. Digital coins worth $60m ($41m) at the time were stolen after the DAO was hacked last year. While the SEC has not decided to make civil charges as a result of its investigation into what happened, it is using the case to caution industry and the market.

A US regulator has said firms looking for backing via crypto-currencies should be more strictly regulated.