The United States has slapped sanctions on a popular cryptocurrency service for helping North Korea launder virtual currency stolen in cryptocurrency hacks.

The U.S. Department of the Treasury announced the sanctions on Monday, saying that virtual currency mixer Tornado Cash was used to launder more than seven billion U.S. dollars in virtual currency since its launch in 2019.

The department said that Tornado Cash provided the means to illicitly transfer over 455 million dollars stolen by North Korea's state-sponsored hacking group, known as the Lazarus Group.

The department said that the platform was also used to launder 96 million dollars in the June hack of the blockchain tech firm Harmony and at least seven-point-eight million dollars from last week's hack of U.S. crypto startup Nomad.

Monday’s sanctions will freeze the firm's assets in the U.S. and prohibit Americans from using the virtual currency mixer.