Japan's Nintendo Co. may lose a window of opportunity to introduce its hit mobile game app, Pokemon Go to China, experts said on Sunday, as knockoffs are eroding its access to the world's largest game market.

The top free game in China on the Chinese iOS App Store over the weekend was City Spirit Go, a Chinese-developed mobile game app that has been widely perceived as a copycat version of Pokemon Go.

The operators of the Chinese app had to increase the number of servers by ten in just three days to meet increasing demand, news portal sohu.com reported on Wednesday. 

City Spirit Go duplicates several features of the Japanese game, such as being location-based, but there is no augmented reality built in yet, allowing players to discover and capture virtual spirits all over the cartoon map. It was launched in March, a few weeks after Nintendo rolled out a Japanese-only beta test of its Pokemon Go game.

The augmented reality mobile game "Pokemon Go" by Nintendo is shown on a smartphone screen in this photo illustration taken in Palm Springs, California US July 11, 2016. (SAM MIRCOVICH)

The developer of the copycat, Shenzhen Tanyu Interactive Technology Co, couldn't be reached for comment over the weekend.

Chinese gamers' appetite for Pokemon Go appears to be huge. The game had amassed over 279,000 fans on the country's leading online group discussion platform Baidu Tieba by press time on Sunday. 

Pokemon Go is taking the world by storm as well, although it is now only available in Australia, the US, the UK, Germany and New Zealand. Still, it is unclear when Nintendo will bring the game to China mainland although there are reports that a release in Hong Kong are “looming” according to local media. 

China's mobile games market generated $7 billion revenue in 2015, becoming the largest for mobile games worldwide, according to market research firms Newzoo and TalkingData.

A gamer uses the Pokemon Go application on his mobile in the entrance of the fish market in Kuwait City on July 14, 2016.

In June, the State Administration of Press, Publication, Radio, Film and Television ordered every mobile game to gain its approval before publishing in China. Also, Google's mapping service, on which the game is significantly based, is blocked in the mainland, forcing Nintendo to make more efforts to seek a suitable mapping service provider in the market, according to She Shuanglin, a Beijing-based independent virtual reality expert.