The United States placed sanctions Monday on Tornado Cash, a leading "crypto mixer" for transactions in virtual currency that US officials describe as a hub for laundering stolen funds, including by North Korean hackers.

The Treasury said Tornado Cash had been used to transfer at least $96 million of funds stolen in June from crypto exchange service Harmony Bridge, and another $7.8 million of the nearly $200 million in crypto currency hacked from Nomad, a similar service.

In addition, Tornado Cash was used to transfer and mask $455 million of the more than $600 million worth of ethereum, a leading virtual currency, stolen in April from the Axie Infinity game via the Ronin Network.

That theft, which the Treasury called the largest known crypto heist to date, was carried out by North Korean state-backed hacking units known as the Lazarus Group and APT38, according to the US Federal Bureau of Investigation.

Tornado Cash is one of the leading mixers, also known as tumblers, which help people hide the movements of their crypto currencies like bitcoin and ethereum by blending them with the transfers by other users.

Tornado Cash "has been used to launder more than $7 billion worth of virtual currency since its creation in 2019," the Treasury said.

In May the Treasury sanctioned another large crypto mixer, Blender.io, which it said was also used to process crypto funds stolen by the North Korean groups.

US officials say the mixer services aren't themselves illegal but are frequently used to move illicit funds, and that other mixers and anonymity-enhancing technologies in the crypto ecosystem are under scrutiny.

The sanctions prohibit US individuals or firms from using Tornado Cash and would risk losing those funds if they did.