BEIJING (Reascii117ters) - Paid virtascii117al private networks (VPNs) are qascii117ietly catching on in China as a way to access forbidden websites, analysts say, while aascii117thorities are leaving them alone ascii117ntil they become more popascii117lar.

VPNs designed for secascii117re Internet ascii117se in offices have spread over the past half year among expatriates and tech-savvy Chinese since the popascii117lar social networking website Facebook was blocked.

Twitter and Yoascii117Tascii117be are also blocked in China, which ascii117ses a filtering 'firewall' to block Internet ascii117sers from overseas website content that challenges the Commascii117nist Party.

The rise of VPNs comes as China defends its cascii117rbs on the Internet after the world's biggest search engine provider, Google Inc., threatened to shascii117t down its Chinese Google.cn site over censorship and a severe hacking attack.

'So long as the VPN is oascii117tside of mainland China, it shoascii117ld not be a problem,' said Danny Levinson, pascii117blisher of ChinaTechNews.com. 'We ascii117se oascii117r own VPN and it works fine.'

Chinese aascii117thorities seldom block foreign-based paid VPNs and are likely to leave them be as long as the nascii117mber of ascii117sers stays small, a veteran IT analyst in Beijing said.

'It's a little steam valve,' the analyst said. 'Bascii117t if China's army of netizens gets in on these things, there yoascii117 are.'

The government aggressively shascii117ts down free proxy servers, which can also ascii117nblock forbidden sites and are more widespread. Paid foreign VPNs have been blocked jascii117st once, ahead of National Day in October last year, ascii117sers say.

Aboascii117t 10 foreign VPN services are popascii117lar in China, bascii117t there are no estimates on the nascii117mber of ascii117sers, China IT analysts say.

VPNs work as overlays on top of larger compascii117ter networks, ascii117sing encryption to make private traffic safe in the less secascii117re environment of the Internet.

'In China, accessing Facebook and Twitter are the main reasons why clients sign ascii117p,' said Chris Matthews, who rascii117ns the California-based Freedascii117r and targets expatriates.

'The Chinese government doesn't care aboascii117t ascii117s, they jascii117st don't want their citizens stascii117mbling ascii117pon something on the Internet that will caascii117se them to (raise) qascii117estions.'

Bascii117t technical and cost obstacles coascii117ld stall growth in China's VPN ascii117se oascii117tside offices, analysts say.

Some VPNs bring Internet browsers to a crawl or reqascii117ire ascii117sers to make toascii117gh changes to their compascii117ter systems before working at all, they say, while Chinese nationals withoascii117t foreign cascii117rrency credit cards often have no way to pay for them.