Popular Asian messaging app Line is now blocking 535 words for its users in China.

University of Toronto's Citizen Lab researchers reported that Line has added around 300 words to the sensitive keywords banned in China, which include references to the 1989 Tiananmen Square violence and other "prurient" phrases, according to Quartz.

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In November, Citizen's Lab discovered that the popular chat platform had activated a censoring function for mobile handsets registered to China-based numbers, the report said.

Quartz quoted Line saying that the self-censoring had to be done to comply with the local rules as it expands into mainland China.

Line's exclusions of words associated with past and low-profile scandals seems to indicate that instead of just following the pre-set rules, the Japanese firm is also implementing the censorship independently. This practice is common among media and tech firms in China, the report explained.

Included in the list of banned keywords are those linked with former Chinese official, Bo Xilai, who was involved in a controversial and widely-discussed downfall. References to Chinese leader Jiang Zemin's rumored death in 2011 are also included in Line's censored list, Quartz said.

According to the researchers, the inclusion of low-profile incidents in Line's ban list indicates that the self-censorship is a "pre-emptive" and "preventative" move, the report said.

When compared with the terms blocked in other popular messaging apps being used in China, Line's list of blocked words only have a little in common. This means that the censoring behaviors of messaging platforms in China are rather inconsistent and arbitrary, Quartz continued.

Out of the 535 words that Line included in its ban list, only 45 words matched those that are banned on Skype in the country. In addition, only 19 words from that list matched that of Sina-owned platform Sina UC, the report stated.

The Citizen Lab researchers said the low percentage of overlap between ban lists in the country suggest that there is no common list of banned keywords provided to firms that run chat apps in China, Quartz reported.