Trump's bitcoin comments appear to have been sparked by unease around social media giant Facebook's planned Libra cryptocurrency, which the chair of the U.S. federal reserve, Jerome Powell, this week said: "raises many serious concerns."

"I am not a fan of bitcoin and other cryptocurrencies, which are not money, and whose value is highly volatile and based on thin air," Trump said. "Unregulated crypto-assets can facilitate unlawful behavior, including drug trade and other illegal activity."

The bitcoin price has swung wildly over the last few years, rising from under $1,000 per bitcoin to almost $20,000 over the course of 2017 and then plummeting to around $3,000 last year.

Trump, who added "Facebook Libra’s 'virtual currency' will have little standing or dependability," said that Facebook will have to comply with U.S. and international banking regulations if it wants to "become a bank."

"We have only one real currency in the U.S.A., and it is stronger than ever, both dependable and reliable," Trump said. "It is by far the most dominant currency anywhere in the world, and it will always stay that way. It is called the United States dollar!"

Trump's comments could mean Facebook's planned bitcoin rival, which Facebook hopes to launch next year, will bring fresh regulations to the bitcoin and crypto space, with the Fed's Powell this week warning existing rules do not fit digital currencies.

"Libra raises many serious concerns regarding privacy, money laundering, consumer protection, and financial stability,” Powell warned, adding he doesn't think "the project can go forward" until these are addressed.

Facebook, which has been struggling to pivot to a more privacy-based business model in light of its ongoing data-sharing scandal, revealed plans for a global cryptocurrency last month and has recruited 28 partners, including the likes of Uber, PayPal and Mastercard to help govern the bitcoin rival.

Meanwhile, the U.S. so-called alt-right, some of whom are interested in bitcoin and cryptocurrencies as an alternative to the traditional financial services industry after companies refused them service, have been perturbed by Trump's comments.

"This is a major mistake on your part and shows a lack of vision," far-right commentator Mike Cernovich tweetedin a reply to Trump, adding: "Today, bitcoin became mainstream."

Earlier this year, Alex Jones, a Trump fan, bitcoin true believer, and alt-right radio host most famous for being sued by the parents of Sandy Hook school shooting victims, told podcaster Joe Rogan "cryptocurrencies are the future."

Trump's attack on bitcoin and cryptocurrencies comes shortly after he hosted far-right social media provocateurs including Project Veritas founder James O’Keefe, pro-Trump activist Bill Mitchell, and college Republican activist Charlie Kirk, at the White House yesterday for a "social media summit" to discuss "opportunities and challenges" of the internet.