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China has blocked several popular services that let citizens skirt state censorship systems.

Three providers of Virtual Private Network (VPN) systems reported that updates to China's firewall had hindered people using their services.

The providers affected are Astrill, StrongVPN and Golden Frog.

Many Chinese people use VPNs to visit websites outside the country that they would not be able to reach without the aid of such tools.

Sites blocked in China include services operated by web giants such as Google, Facebook and Twitter.

China operates a very sophisticated net censorship system that both limits the places people can go online and what they can search for and discuss.

A VPN works by setting up a dedicated, encrypted link between a person's computer and the website or service they want to use and makes spying on the data flowing across the connection difficult.

Chinese state media said the blocks had been imposed "for safety". Reuters reported that a cybersecurity expert at a state-backed think tank said the upgrades to the nation's firewall had been carried out to preserve China's "cyberspace sovereignty".

The renewed attempt to stifle use of VPNs comes as the ruling Communist party seeks to clamp down on corruption by top officials, Prof Xiao Qiang from Berkeley's School of Information told AP.

The clampdown was "a very clearly related fact with the amount of political rumours and information related to China's high politics showing up in websites outside of China,'' he said.

The services that have been hit are almost exclusively used by individuals and are often accessed via mobile phones. China has not put any restrictions on the use of VPNs inside large corporations.

Sunday Yokubaitis, president of the Golden Frog VPN service, told Reuters: "This week's attack on VPNs that affected us and other VPN providers is more sophisticated than what we've seen in the past."

Despite this, Golden Frog said access to some of its servers was still unimpeded.

StrongVPN that it was "working diligently" to restore access to servers it had in China. In addition, it said customers should attempt to connect at non-peak hours to limit the load on its network.