Apple is removing software from its app store in China that allows users to skirt the country's Great Firewall.

The operators of two virtual private networks, or VPNs, said Saturday that they had been notified by Apple (AAPL) that their products were being taken down.

"We are writing to notify you that your application will be removed from the China App Store because it includes content that is illegal in China," Apple said in a notice to ExpressVPN. "We know this stuff is complicated, but it is your responsibility to understand and make sure your app conforms with all local laws."

A second service, called Star VPN, said in a tweet that it had received a similar notice.

We just received notice that @Apple removing all the @VPN apps from the @China app store. — Star VPN (@star_vpn) July 29, 2017

China's internet is tightly controlled by censors, who block access to services such as Google (GOOGL), Facebook (FB) and even the New York Times (NYT). To get around the firewall, many people use VPNs, which use encryption to disguise internet traffic.

Beijing said in January it would begin to restrict VPNs, and this month reportedly told the country's three big telecom companies to block individuals' access to them by early next year.

"We have been required to remove some VPN apps in China that do not meet the new regulations," Apple said in a statement Sunday.

Related: China says VPN crackdown aimed at 'cleaning' the internet

China's internet regulator defended the crackdown earlier in the week, saying recent measures were part of an ongoing campaign aimed at "cleaning and standardizing" access to the internet.

"Our restrictions target service providers without licenses or operating illegally," Zhang Feng, a spokesman for the Ministry of Industry and Information Technology, said at a press conference on Tuesday.

"Law-abiding individuals and businesses won't be affected," he added.

Authorities have cracked down on everything from Facebook's popular messaging app WhatsApp to pictures of Winnie the Pooh in recent weeks.

Related: China fortifies Great Firewall with crackdown on VPNs

ExpressVPN said that it was "disappointed" and "troubled to see Apple aiding China's censorship efforts."

"Our preliminary research indicates that all major VPN apps for iOS have been removed," the popular service said. It added that Chinese users could still access its app on other platforms including Android and Windows.

China is Apple's largest market outside the United States, and the company has revoked access to the App Store at the behest of Chinese authorities before. Multiple apps belonging to the New York Times were removed from the store last December.

The tech firm, which sells a huge number of iPhones in China, has occasionally been on the receiving end of Chinese censorship, with its iBooks and iTunes Movies services being shut down last year.

The U.S. views China's censorship as a barrier to trade, arguing in an annual report that China's "extensive blocking of legitimate websites" imposes significant costs on suppliers and users of services and products.

China has tightened internet censorship across the board ahead of a key Communist Party meeting this fall, where a major senior leadership reshuffle is expected.

-- Steven Jiang and Nanlin Fang contributed reporting.