BEIJING, March 1 (Xinhua) -- Chinese gaming giant Tencent has further tightened restrictions on underage game players by imposing a digital lock on logging onto some games.

Players under 13 years old have to ask their guardians to unlock the game, the latest step from Tencent to avoid game addiction in children, the company announced Friday.

The new restriction will be gradually piloted for the Chinese versions of its two popular games "Honor of Kings" and "PUBG Mobile" in 12 Chinese cities including Beijing, Chengdu and Changchun.

The company has introduced a string of systems in the past two years to ensure that minors play games in a healthy manner.

Tencent decided in 2017 that players under the age 12 could only play most of its games for one hour at most each day and are banned from playing from 9 p.m. in the day to 8 a.m. the next morning.

China is the world's largest gaming market, with sales reaching 214.44 billion yuan (about 32.01 billion U.S. dollars) last year, up 5.3 percent year on year, according to the China Game Publishers Association Publications Committee.