US tech giant Microsoft has confirmed that its search engine Bing is currently inaccessible in China.
Social media users have expressed concern that the search engine might be the latest foreign website to be blocked by censors.
Chinese authorities operate a firewall that blocks many US tech platforms, including Facebook and Twitter.
Microsoft hasn't said if the outage may be due to censorship, or is merely a technical problem.
"We've confirmed that Bing is currently inaccessible in China and are engaged to determine next steps," Microsoft spokesperson said in a statement.
A BBC correspondent in China attempted to visit the site, and was able to access it through a Chinese internet provider on a desktop, but not on a smartphone.
Many US tech companies are keen to tap into the Chinese market, but have a difficult relationship with the authorities in Beijing.
The government's internet censorship regime, often known as the "Great Firewall", uses a series of technical measures to block foreign platforms and controversial content.
Chinese authorities have also cracked down on Virtual Private Networks, which allow users to skirt around the firewall.
China-based messaging services and social media are restricted, with key words and expressions blocked if they express dissent or ridicule senior political leaders.
Bing's rival Google shut down its search engine in China in 2010, after rows with the authorities over censorship and hacking.
Google has said that it has no immediate plans to re-launch a search engine in China, but has admitted it has looked closely at the idea.
Although Twitter is blocked, it maintains a Greater China office because Chinese customers can use the platform to advertise abroad.
Facebook attempted to set up an office in China last year, but appears to have been blocked.
Microsoft has maintained an office in Beijing since 1992. It has continued to operate Bing and its communication service Skype in China.