North Korea has apparently blocked access to Instagram within its borders, jeopardizing one of the most popular ways for journalists and foreigners to document life in the hermetic country. As the Associated Press reports, users who open Instagram on the Koryolink 3G network have reported seeing a message that reads: "Warning! You can't connect to this website because it’s in blacklist site." A warning in Korean adds that the site contains "harmful content."

The AP reports that the photo-sharing app was still usable on some mobile devices despite the warning, but posting photos or viewing user profiles was impossible on other devices connected to the 3G network. The warnings have also appeared when accessing Instagram on desktop computers with LAN cable connections.

It's not clear what spurred the apparent blacklisting. Koryolink, North Korea's only 3G operator, says it wasn't notified of a change in policy, and the government has yet to issue a statement on the matter. There is speculation that the Instagram block may be a response to a fire that erupted this month at a popular hotel in Pyongyang. The incident wasn't reported by North Korea's state-run media, though images spread across social media.

North Korea is among the least-connected countries in the world, with internet access largely limited to top government officials, and strict censorship policies. But it has relaxed its controls for foreigners, who since 2013 have been able to access web on the country's 3G network and from their own cellphones. Social networks like Twitter and Facebook (which owns Instagram) remain accessible within North Korea, despite the apparent crackdown on Instagram. A recent activist mission led by Gloria Steinem was even live-streamed from Pyongyang over Periscope.

For outsiders, Instagram has provided an important window into one of the world's most secretive countries. In recent years, a handful of tour guides, journalists, and other Instagram users have amassed major followings on the social network, with photos of everyday life that add an important counterweight to the propaganda published by the regime of Kim Jong-un.

"Most people don’t know anything about North Korea, in part because there's never been an independent visual record of the country," says David Guttenfelder, a photojournalist and National Geographic fellow who shot critically acclaimed Instagram photos of North Korea while working there for the AP. Last year, he launched the Instagram account Everyday DPRK, which, like Everyday USA, collects images from a wide spectrum of photographers including a teacher, a tour operator, and Guttenfelder himself.

"There’s no better place to test the power of photography and social media than a place like North Korea," Guttenfelder says. "The photography that has been posted has been a really important part of illuminating a country that we don’t know anything about... I think it's a shame if they aren't able to continue to do it."