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Nintendo has released its highly-anticipated debut smartphone game, Miitomo, in Japan.

The app is a social game in which players create an avatar known as a Mii and share posts with friends from Facebook and Twitter.

Players are encouraged to answer questions about their likes and dislikes and the answers can be seen by friends in the app.

Early reviews of the app have been mixed.

Video game site IGN said Miitomo had "a lot of potential" but had "missed opportunities with the lack of room customisation".

Technology site The Verge said Miitomo was "a little clunky at times, with too many load screens" but did have an abundance of "sleek, appealing quirkiness".

But the first 500 reviews on the Japanese version of the iOS app store have been largely positive.

Nintendo first announced its intention to produce smartphone games in March 2015, after widespread demand for its best-selling franchises to be made available on mobiles.

Miitomo lets players dress up their avatar with Nintendo-themed accessories and share pictures of their Mii with friends.

"It's a new kind of social network, and it's quite strange," said Alex Olney, a games journalist at .

"You are questioned by the app about what you did at the weekend and what food you like and the answers are relayed to your friends. It's strangely engrossing."

Although the game offers in-app purchases, where players can part with real cash to buy in-game currency, Mr Olney said he felt like the app was more likely to be a "gateway" into the Nintendo ecosystem.

"It links to a Nintendo account and Miitomo can be used to get rewards. Save enough and you might get a discount or free downloadable content for a console game," he told the ����ý.

"It gets people on to their platform - a gateway into the wider Nintendo world."

Nintendo's social software has not always been a success. In 2013, it closed the Letterbox messaging service on its 3DS console over concerns people were sharing indecent images on the platform.

Despite the closure, Mr Olney said Miitomo did not have content filters built into it.

"There is no language or photo filter, it is something we brought up with them," he told the ����ý.

"But it is a closed circle of friends - you can't add people just by email address, you have to be friends with them on Facebook or follow one another on Twitter. And you can report content you find inappropriate."

Miitomo, which is expected to be released in the UK in March, is the first of five mobile games Nintendo is producing with Japanese firm DeNA.

In January, Nintendo's president Kimishima Tatsumi said he hoped to use some of the firm's famous characters in smartphone games in the future.