SHANGHAI - China's bike-sharing bubble is showing signs of strain, after the country's third largest firm said it was going under.

The country has seen a wave of iconic, brightly coloured shared bicycles hit its city streets over the last year, helping revolutionise urban travel, but also drawing some public ire over mountains of bike clogging up sidewalks.

The chief executive of Bluegogo, Mr Li Gang, said in a public letter posted online last week (Nov 16) that he had "made mistakes" and the firm was winding up.

He apologised to investors, partners and 20 million registered users of the company's 830,000 blue bikes.

"I was filled with arrogance," he admitted, adding he had endured months of sleepless nights.

He said financing in the market had become a "bubble", reported Reuters.

China's bike-sharing craze has been driven by huge investments, especially into the two market leaders Mobike and Ofo, which have raised billions of dollars from tech giants like Tencent Holdings and Alibaba Group Holding Ltd.

The two brands - often seen as China's "Uber for bikes" - have deployed millions of bicycles around the country, and pushed overseas into markets in the United States and Europe.

Following Bluegogo's collapse, many users have complained of difficulty getting refunds of their deposits.

Users of another Chinese bike-share company, Coolqi, also complained of similar problems after the company went bust this month, reported Xinhua news agency on Thursday (Nov 23).

Coolqi, which had 1.4 million bikes, announced on Monday that it has entrusted a company in Chengdu, capital of southwest China's Sichuan Province, to deal with refunds.

For now, both Ofo and Mobike remain afloat, supported by some of the world's biggest and most generous funders. But the pressure to generate cash is growing and as recently as October they were in talks to merge, reported Bloomberg.

Meanwhile, photographer Chen Zixiang has captured images of a huge bicycle graveyard in the south-eastern city of Xiamen, reported The Guardian.

The towering pile on an area roughly the size of a football pitch had thousands of bikes from Mobike, Ofo and Bluegogo. It appears to be one of the largest amalgamations of discarded bicycles from around the city, the report said.