South Korean-based site accepted digital currency for access to videos, with victims rescued in US, UK and Spain.

Hundreds of people have been arrested in a worldwide operation over a South Korea-based dark web child sexual abuse site that sold videos for digital cash.

Officials from the United States, Britain and South Korea described the network as one of the largest operations they had encountered to date.

Called Welcome To Video, the website relied on the bitcoin cryptocurrency to sell access to 250,000 videos depicting child sexual abuse, authorities said, including footage of extreme abuse of young children. Its upload page specifically stated, “Do not upload adult porn”.

“Darknet sites that profit from the sexual exploitation of children are among the most vile and reprehensible forms of criminal behaviour,” US assistant attorney general, Brian Benczkowski, said.

Officials have rescued at least 23 underage victims in the United States, Britain and Spain who were being actively abused by users of the site, the justice department said. Many children in the videos have not yet been identified.

The site’s vast library – nearly half of it consisting of images never seen before by law enforcement – is an illustration of what authorities say is an explosion of sexual abuse content online. In a statement, Britain’s National Crime Agency said officials were seeing “increases in severity, scale and complexity”.

Welcome To Video’s operator, a South Korean named Jong Woo Son, 23, and 337 users in 12 different countries, have been charged so far, authorities said.

Son, currently serving an 18-month sentence in South Korea, was also indicted on federal charges in Washington.

Several other people charged in the case have already been convicted and are serving prison sentences of up to 15 years, according to the justice department officials.

Welcome To Video is one of the first websites to monetise child sexual abuse using bitcoin, which allows users to hide their identities during financial transactions.

Users were able to redeem the digital currency in return for “points” that they could spend downloading videos or buying all-you-can watch “VIP” accounts. Points could also be earned by uploading child sexual abuse images.

“These are the bottom feeders of the criminal world,” said Don Fort, chief of criminal investigation at the US internal revenue service (IRS), which initiated the investigation.

The justice department said the site collected at least $370,000 worth of bitcoin before it was taken down in March 2018 and that the currency was laundered through three unnamed digital currency exchanges.

Fort said the investigation was triggered by a tip to the IRS from a confidential source. However, Britain’s National Crime Agency said it came across the site during an investigation into a British academic who in October 2017 pleaded guilty to blackmailing more than 50 people, including teenagers, into sending him images that he shared online.

In a statement, British authorities said the National Crime Agency’s cybercrime unit deployed “specialist capabilities” to identify the server’s location.

The justice department gave a different explanation, saying that Welcome To Video’s site was leaking its server’s South Korean internet protocol address to the open internet.