China's hugely popular social media app WeChat operates different censorship systems for Chinese- and overseas-based users, a research group found, applying the Communist country's rules to mainland-registered accounts even when their owners are abroad.

WeChat, known as Weixin in China, is the world's fourth most popular social media platform, according to the University of Toronto's Citizen Lab, with 806 million monthly active users.

More than just a chat application, it offers payment, ride-hailing and other services, and owner Tencent has ambitions to spread it far beyond its home country.

In China, Communist authorities censor online content they deem politically sensitive, while blocking some Western websites and the services of Internet giants including Facebook, Twitter and Google with a vast control network dubbed the Great Firewall of China.

WeChat censors content for all users registered with Chinese phone numbers, Citizen Lab found, even if they travel overseas or switch to an international number.

Censorship for accounts tied to overseas numbers was limited to the intermittent blocking of gambling or porn-related content within the app's internal browser.

"WeChat has seemingly created a 'One App, Two Systems' model for censorship," said the report, posted on the group's website Wednesday.

Foreign companies such as LinkedIn have agreed to censor their content in exchange for access to the world's second-largest economy and most populous country, a potentially lucrative market.

Facebook, banned on the mainland since 2009, has reportedly built a software tool for geographically censoring posts as it seeks access.

Citizen Lab identified 174 keywords and phrases across four languages -- English, Uyghur and traditional and simplified Chinese -- that triggered censorship, noting that the list changed over time in response to events.

Messages containing such words sent by mainland-registered users disappeared without notification to either sender or would-be receiver, Citizen Lab found, adding that more keywords were blocked in group chats than in one-on-one conversations.

More than half the tested phrases -- such as "don't forget June 4th" -- were related to the bloody 1989 crackdown in Tiananmen Square.

Another 13 percent were related to Communist party officials, it added, giving examples such the term "Xi-tler", an unfavourable nickname comparing President Xi Jinping to Adolf Hitler, and rumours such as the supposed death of former head of state Jiang Zemin.

Tencent, which became China's most valuable company in September, told AFP that it "respects and complies to local laws and regulations" in its countries of operation and sought to provide WeChat users "high-quality, secure" user experiences.