Update: The Youtube channels Stimmekoreas and Uriminzokkiri have since been reinstated by Google since this article was published.

YouTube removed several major North Korea-related channels on Friday “for violating YouTube’s Community Guidelines”, notices on the former channels reveal, amid a broader U.S. crackdown on the country following its sixth nuclear test.

The channels all regularly published content related to North Korea or videos of North Korean state media-produced content, and were used extensively by researchers following the country on a day-to-day basis.

“The channel was 7 years old, it was a great way to watch the DPRK’s point of view over a long period of time,” the unnamed administrator of “Stimmekoreas” – one of the channels removed – told NK News on Friday. “The whole transition of power from Kim Jong Il to Kim Jong Un was documented on this channel.”

Stimmekoreas was known as one of the most popular North Korea-related YouTube channels and in 2014 had registered over “15 million views for the hundreds of videos it hosted,” according to Martyn Williams, author of the North Korea Tech blog.

YouTube did not provide the administrator of Stimmekoreas a reason for the takedown, beyond a notice that they were no longer able “to access a Google product.”

Other channels removed on Friday include “Tonpomail” and “Uriminzokkiri”, which also posted daily North Korea-focused content.

Uriminzokkiri is directly tied to the North Korea propaganda apparatus and, while its YouTube channel has been removed, its other social media pages on Flickr, Twitter, Google + and Tumblr are still active.

An apparent effort to remove North Korea focused channels by YouTube appeared to begin in late 2016, when the website took down the KoreaCentralTV1 channel that claimed to be an official representative of North Korea’s state tv channel.

Several other video channels were removed including “Chosun TV”, “KCTV Stream,” and “NK Propaganda,” among others.

At the time the KoreaCentralTV1 channel was deleted from YouTube, North Korea Tech’s Williams commented that the suspension may have been designed to prevent North Korean administrators profiting from advertising revenue, a transaction that would have breached U.S. Treasury restrictions introduced in 2016.

In January, administrators of two North Korea-focused channels which had been removed told NK News that they had received no warning or reason from YouTube when the same action was being taken against their channels.

Neither of these channels or their administrators were tied to the North Korean government.

While YouTube provides little reasoning behind decisions and has not responded to NK News comment requests, the removal of such content is a cause for concern for North Korea watchers and researchers.

“While I don’t understand YouTube’s motivations for this move, it is incredibly frustrating for researchers who use North Korean propaganda for analytic purposes,” Scott Lafoy, a Washington-based satellite imagery analyst, told NK News on Friday.

“Tracking and digitally reconstructing events is going to be more difficult as these accounts get deleted,” he added. This was a sentiment echoed by the administrator of Stimmekoreas.

“Every major event of the last (few) years was covered. For DPRK researchers it was a great tool.”

Researchers of other subjects have also run into similar problems with YouTube and the resulting loss of open source material.

“YouTube is already under criticism for similar terminations of accounts with Syrian Civil War and ISIS related content, they aren’t exactly making it easy for open-source researchers,” Lafoy said.

“Ironically, this is going to drive up traffic to Youku and other foreign competitors that still host DPRK-related content.”

Additional reporting by Leo Byrne