BEIJING: Chinese authorities, who have long sought to limit access to information, seem to have reinforced the so-called Great Firewall (GFW) of China, frustrating foreign companies and raising activists’ concerns.

The GFW—the country’s huge system of Internet limits and censorship—now appears to be stepping up targeting of virtual private networks, or VPNs, commonly used to circumvent controls on websites the government considers threatening.

While VPNs let users gain access to sites blocked by the Firewall due to their content or sensitivity, which in China include Facebook and Twitter, they are also vital to firms, enabling secure and encrypted communications.

In the globalized economy companies have flocked to China to try to participate in decades of stunning growth. But web users are complaining of VPNs being inaccessible or quickly going down once accessed, while speeds have slowed to a crawl.