Microsoft confirmed Thursday that Bing was unavailable in China, raising concerns that it could be the latest in a growing list of global internet platforms to be shut out of China's huge market.

Hours later, however, some users were once again able to access the service.

"We can confirm that Bing was inaccessible in China, but service is now restored," aspokesperson told CNN Business on Thursday.

Bing is the last major foreign search engine operating in China after pulled out in 2010 . The service interruption suggested that even tech companies that submit to Beijing's strict internet censorship regime can still run into trouble in the country.

Microsoft's potential setback comes as China and the United States are locked in a widening confrontation over technology and access to each other's markets that experts warn could be the start of an economic cold war.

It wasn't immediately clear why Bing was being blocked. China's internet regulator didn't respond to a request for comment Thursday, and the Foreign Ministry declined to comment.

Chinese users first noticed problems with the search engine late Wednesday, when the phrase "Can't access Bing" started popping up on social media.

"It's not the first time that we've encountered issues like this for Bing in China, these do arise periodically," Microsoft President Brad Smith said during a panel at the World Economic Forum in Davos , Switzerland.

Apple AAPL Microsoft, which recently overtookas the world's most valuable company, has encountered problems in China before. The company's internet video and phone call platform Skype was pulled from Apple and third-party Android app stores in China in November 2017.

The Bing blackout comes as Chinese President Xi Jinping's government seeks to tighten its control of the internet using the vast censorship apparatus known as the Great Firewall.

On Wednesday, the same day Bing started to become inaccessible for users, the Cyberspace Administration of China announced that it had closed 733 websites and shut down 9,382 apps in a crackdown on "harmful" information.

"Over the past couple months, it appears the Chinese government has improved [the] accuracy and frequency of the Great Firewall blocking," said Sunday Yokubaitis, CEO of internet services company Golden Frog.

"The Great Firewall is only getting better," he added.

Smith said Microsoft isn't certain whether the blocking of its search engine "is confined to Bing or if it is something that is broader."

Playing by Beijing's rules

Facebook FB Twitter TWTR Top US internet platforms likeand YouTube have been blocked for years.

Chinese censors crack down on content and conversations about subjects deemed sensitive by Beijing, including the Tiananmen Square massacre, Tibet and criticism of President Xi. They also punish sites that fail to weed out material like pornography and crude content that the ruling Communist Party considers vulgar or otherwise harmful to society.

Bing was able to operate its Chinese site, cn.bing.com, because it censored its search results.

Microsoft is part of the Internet Society of China, a government-linked body whose members pledge to refrain "from establishing links to websites that contain harmful information" or share any content which could "jeopardize state security and disrupt social stability, contravene laws and regulations and spread superstition and obscenity."

Smith said Microsoft has "days when there are either difficult negotiations or even disagreements" with Chinese authorities about search results on Bing.

"But we're not aware of any ongoing negotiation or disagreement, so we're working to understand it better," he added, referring to its sudden inaccessibility.

Human rights groups have repeatedly criticized international companies for adhering to Beijing's strict censorship rules. But China has more than 770 million internet users and a thriving online shopping market, making it impossible for top tech companies to ignore.

censored Google.cn service. Google came under fire last year when news emerged that it was planning to launch a censored version of its search app in China . The company effectively left China in 2010 when it stopped running itscensored Google.cn service.

Alibaba-backed BABA Chinese tech firm Baidu is the dominant player, accounting for 70% of the market last year, according to research firm StatCounter.Shenma is the second biggest with about 16%.

Bing has struggled to make significant inroads in China, capturing just 2% of the search market.

"Bing has a tiny market share in China, but appearance matters to Beijing," said Lokman Tsui, associate professor at the Chinese University of Hong Kong and former head of free expression at Google in Asia.