Cryptocurrencies are developing so rapidly that they may soon threaten the stability of the global financial system, according to Klaas Knot, president of Dutch central bank and international body Financial Stability Board.

“We have long maintained that crypto does not yet pose a systemic risk, but recent developments suggest we may be nearing a tipping point,” Knot said on Wednesday at a meeting of FSB officials in Madrid.

Knot, who is also stepping down from both roles this month, said he is especially concerned about crypto companies that issue so-called stablecoins — digital currencies designed to maintain a steady value by tying their worth to other stable assets such as government bonds, commodities or major currencies.

One of his main concerns, he said, is the increasing presence of US government debt in the portfolios of stablecoin issuers. “These companies now hold a significant amount of American debt securities. This is something we must monitor closely,” Knot is quoted as saying by BNR radio.

In the United States, lawmakers are currently working on regulations for stablecoins. But the process has raised eyebrows, because the crypto industry itself is largely supportive of the proposed rules.

Several officials at the European Central Bank have voiced concern over the potential consequences and the ECB warned in a recent report that stablecoins could present financial risks, due to their growing ties with traditional financial markets.

Knot also pointed to the rising popularity of cryptocurrency investment among ordinary consumers.

He said the launch of crypto-based ETFs has significantly lowered the barrier for retail investors. “The integration with the traditional financial system continues to increase. The crypto ecosystem will keep evolving, and so must our regulatory frameworks,” he said.

Knot will be replaced at the FSB Britain’s Andrew Bailey, director of the Bank of England. His successor at the central bank has not yet been announced.