Developers of virtual private network (VPN) apps said Apple has removed their apps in China in another sign its government is clamping down on Internet access.

Two companies -- ExpressVPN and Star VPN -- revealed on Twitter their apps were pulled from Apple's App Store in China.

In a blog post, ExpressVPN said "preliminary research" confirmed all major VPN apps were removed in China.

"We're disappointed in this development, as it represents the most drastic measure the Chinese government has taken to block the use of VPNs to date, and we are troubled to see Apple aiding China's censorship efforts," wrote ExpressVPN.

Apple could not be immediately reached for comment. In a statement obtained by TechCrunch, Apple said developers of VPN apps must get a government license to operate. "We have been required to remove some VPN apps in China that do not meet the new regulations," the statement said.

VPN services allows users to securely access the Internet when connected to WiFi hotspots. It also lets users access websites that may be blocked. Facebook, Google and YouTube are among the most notable services banned in China.

The disappearance of these apps is part of a wider crackdown China has implemented against VPNs. Two years ago, China blocked VPN access to the Internet, forcing developers to register with the government.

China is also a critical market for Apple, makers of the popular iPhone. It is the second biggest market for Apple behind the United States, while 7 of the top 10 grossing Apple retail stores were based in China.

More on China and the Internet:

China cracks down on Internet access

Winnie-the-Pooh banned by China, report says

Follow Brett Molina on Twitter: @brettmolina23.