German authorities said Wednesday they had taken down the "world's largest" money laundering site for cryptocurrency, seizing millions of euros' worth of bitcoin in the process.

Law enforcement officials secured around 44 million euros ($46 million) of the digital currency in the move against online platform ChipMixer, the Federal Criminal Police Office (BKA) and Frankfurt prosecutor's office said in a statement.

Working together with authorities in the United States and Europol, investigators also seized the servers belonging to the "darknet" site.

ChipMixer's operators are suspected of "commercial money laundering and running a criminal trading platform on the internet", the German authorities said.

The site is said to have accepted criminally obtained bitcoin to pay it out again after it had undergone "mixing" to conceal its origin.

ChipMixer promised users "complete anonymity", according to investigators.

The service, in operation since the middle of 2017, is estimated to have laundered "around 154,000 bitcoin" worth about 2.8 billion euros, they said.

Much of it came "from ransomware groups and from other criminal activities", officials said.

ChipMixer was "the world's largest money-laundering service on the darknet", they said.

The secret "darknet" includes websites that can be accessed only with specific software or authorisations, ensuring anonymity for users.

German and US authorities have recently worked together to act against criminal activities involving cryptocurrencies.

Last year, authorities in the two countries shut down Hydra, an anonymous, illicit online marketplace, which also operated on the darknet.

Transactions from Hydra worth "millions" were found on ChipMixer, investigators said.