China’s “Great Firewall” internet censorship system has begun disrupting several VPN services that allow users in the country to access blocked websites and apps, according to reports. Astrill, StrongVPN and Golden Frog are the VPNs currently experiencing outages.

“The Chinese government has attempted to curtail the use of VPNs that its citizens use to escape the Great Firewall for a couple years,” Golden Frog President Sunday Yokubaitis told the Associated Press in a statement. “This week’s attack on VPNs that affected us and other VPN providers is more sophisticated than what we’ve seen in the past.”

On its website, Golden Frog recommends that users having connection issues try server locations in Hong Kong or the Netherlands. StrongVPN an Astrill, meanwhile, also report that they are aware of the problems and are working to resolve them.

VPN stands for “Virtual Private Network,” and while technically any group of computers networked together over a public network can be a VPN, the term most often refers to services that secure a computer’s internet connection, encrypt data that is sent and received, and hide that data from spying eyes.

VPNs can be used to bypass censorship controls in countries like China where internet use is severely restricted. Google services, for example, have been periodically disrupted in China since 2010, and have been permanently blocked since this summer, TechCrunch reports.

State-controlled media in China is claiming the this current round of VPN interference is fully justified. According to Reuters, government-backed “cybersecurity experts” say that the country’s censorship system has been “has been upgraded for cyberspace sovereignty.”