Paris Hilton, boxer Floyd Mayweather and others have all publicly backed digital currency funding drives.

The Securities and Exchange Commission (SEC) said the endorsements could break laws on selling securities.

Those promoting crypto-currencies must say if they are being paid for the endorsement, it said.

Cash caution

In its official warning, the SEC pointed to the growing numbers of public figures who have talked about the funding drives, known as Initial Coin Offerings (ICOs), that crypto-currencies run to raise cash.

About 270 separate crypto-cash start-ups have sought funds via ICOs in 2017, said the New York Times. In total, the ICOs have raised more than $3bn (£2.27bn), it said.

The SEC said any virtual coins or tokens bought by investors through an ICO were subject to the same laws governing the sale of stocks and shares sold via mainstream stock markets.

These laws require anyone backing a security to disclose their relationship with the company offering the investment opportunity.

"A failure to disclose this information is a violation of the anti-touting provisions of the federal securities laws," it said, adding that keeping quiet about any financial arrangement could be considered fraudulent.

It said it was keeping an eye on people who promote the crypto-currencies to ensure laws were not broken.

In a separate but related bulletin, the SEC cautioned against following the investment advice of any public figure.

"It is never a good idea to make an investment decision just because someone famous says a product or service is a good investment," it said.

It encouraged investors to do their own research into potential money-making schemes and not rely on paid endorsements, it said.

Regulators in Switzerland, Britain and Malaysia have also issued warnings about the risks associated with ICOs. Financial watchdogs in China and South Korea have gone further and banned them altogether.