China is cracking down on unrestricted access to the Internet by blocking services that allow users to access Twitter, Facebook, Gmail and others services which get around government censorship.

Various foreign-based operators of virtual private network (VPN) services have said that access to their services in China had been disrupted as a result of the widespread crackdown and users are facing a harder time getting to some foreign websites.

VPNs work by establishing an encrypted pipe between a computer or smartphone and a server in a foreign country. All communications are sent inside the pipe, effectively shielding Internet traffic from government filters that determine whether a site can be accessed. Chinese citizens use VPN to get to external news sources and for day-to-day communications.

The VPN provider Golden Frog wrote on its blog that the controls have hit a wide most of the VPN services.

The popular provider Astrill informed its users this week that the controls have started hitting iPhone access to services such as Gmail.

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“The Chinese government has attempted to block the use of VPNs that its citizens use to escape the Great Firewall for a couple years,” wrote Golden Frog President Sunday Yokubaitis 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.”