HONG KONG - The Chinese government has launched a campaign to clamp down on underaged Internet gaming, requiring operators to install software that discourages teenage players from spending more than three hours online.

Under government rules that went into effect Monday, Chinese Internet gaming companies must implement a screening program that requires users to enter their identification card numbers.

The program then monitors the online time of players identified as under 18 and prompts them at the three-hour mark with a message asking them to stop and "do suitable physical exercise."

The software monitor slashes by half the points of underage players if they keep playing beyond three hours, and wipes their points out completely if they stay on more than five hours.

The screening program is part of a government campaign to combat Internet gaming addiction, "clean up the Internet environment" and "promote civilized Internet use," according to guidelines issued by China's General Administration of Press and Publication explaining the software.

About 10 percent of China's more than 30 million Internet gamers were underaged as of the end of 2006, according to a report by the Chinese newspaper National Business Daily reprinted on the Web site of the General Administration of Press and Publication.

The government guidelines don't flat-out denounce Internet gaming, which has become a popular pastime, stating, "measured gaming is good for the brain, but gaming addiction hurts the body."

The explanation says the three-hour cutoff is based on the time it takes to play a game of the strategy chess game Go.

Shanghai-based gaming company The9, which runs the popular "World of Warcraft" game in China, said on its Web site it was scheduled to launch the screening software this past Saturday at midnight. The world's most popular online game has more than 3.5 million players in China.

President Hu Jintao ordered regulators in January to promote a "healthy online culture" to protect the government's stability, according to state media. And he was quoted in state media in April as urging Communist Party leaders to "curb the spread of decadent and backward ideological and cultural material online."

Though China's communist government promotes Internet use, it has also set up an extensive surveillance and filtering system to prevent Chinese from accessing material considered obscene or politically subversive.

The government has also banned local authorities from approving new Internet cafes this year.

A spokesman for another gaming company, Tencent, was quoted as saying in the South China Morning Post Tuesday that youngsters can beat the new screening program by using false identities.

"It's hard to tell online if the player is a teenager or an adult. Many of them can register for the game using someone else's ID or even a fake ID," Song Yang reportedly said.

It wasn't immediately clear if the new restrictions led to a big drop-off in player volume.

A spokesman for The9 said he wasn't immediately available for comment. Tencent didn't respond to an e-mail seeking comment.