Skype has apparently joined the lengthening list of internet tools on China's blacklist, disappearing from download stores with Apple saying on Wednesday that it was ordered by the government to remove certain apps.

Skype is no longer available for download from the Chinese Apple Store or popular Android sites, with Chinese web-users saying it had been gone for weeks.

"We have been notified by the Ministry of Public Security that a number of VoIP (voice over internet protocol) apps do not comply with local law, therefore these apps have been removed from the App Store in China," Apple said in an emailed statement.

"These apps remain available in all other markets where they do business."

The company did not specify which laws such apps were found to have violated.

China has tightened its already stringent online policing this year, including enacting new rules that require tech companies to store user data inside the country as well as imposing fresh restrictions on what is permissible content.

Chinese authorities appeared to severely disrupt the WhatsApp messaging app as they ratcheted up security ahead of a Communist Party congress in October that saw President Xi Jinping consolidate his hold on the country.

The moves have prompted speculation on the Chinese internet that authorities were moving against services with effective encryption, like WhatsApp and Skype, that make them less vulnerable to government monitoring.

The change does not seem to have affected use of previously downloaded versions of Skype, which appeared to work normally.

Also, Skype Business, a separate app tailored for corporate use, was still available for download in China.

Great Firewall

Skype's removal follows Apple's August decision to strike out so-called VPN apps, or virtual private networks, which allow users to skirt the so-called "Great Firewall of China" system of online censorship.

Both cases underscored the dilemma faced by US tech companies seeking to uphold principles of web freedom while holding on to their Chinese market shares.

"We would rather not remove the apps, but like in other countries, we obey the laws where we do business," Apple chief Tim Cook said after the VPN move.

China has for years blocked leading foreign websites or services including Facebook, Twitter, Instagram and a number of news organisations, largely to prevent content that could challenge party rule.

Slamming the door on foreign tech leaders has allowed Chinese champions like Tencent to prosper in the vacuum.

The market value of the Chinese social media and video game giant surpassed that of Facebook on Tuesday as investors sent Tencent soaring into the world's top five biggest firms.

By the end of trading Tuesday, Tencent's Hong Kong-listed shares were worth $523 billion, surpassing Facebook's $519 billion at the time.

China this year also ordered the country's internet platforms to verify the true identity of users before letting them post online content.

The move could make it more difficult to post online anonymously, often one of the only avenues for honest debate in the country.

Skype's removal from app stores comes as China prepares to host its fourth World Internet Conference next month.

The annual event in eastern China is used by Beijing to promote its views about web policy, but has been criticised by rights groups.

On Tuesday, the ruling Communist Party's anti-graft agency said China's former internet czar Lu Wei, who stepped down last year after overseeing a tightening of online censorship, was under investigation for suspected "severe disciplinary violations," which typically means corruption.