Microsoft announced on Thursday that its search engine, Bing, has been blocked in China, the last major foreign search engine operating in the country.
"We've confirmed that Bing is currently inaccessible in China and are engaged to determine next steps," CNN quoted a Microsoft spokesman as saying.
It was not immediately clear why Bing has been being blocked.
China's internet regulator is yet to comment on the development.
Microsoft, which recently overtook Apple as the world's most valuable company, is the latest US tech giant to face difficulties in China.
Top internet platforms like Facebook, Twitter and YouTube have been blocked in China for years, blacked out by a vast government censorship apparatus known as the Great Firewall.
Google pulled out in 2010.
According to Greatfire.org, a group that tracks what sites are blocked in China, said the site appeared to be inaccessible in parts of the country but reachable in others, reports The New York Times.
China's blockages often take time to spread nationwide, though in the past some services have been blocked in some places only to be restored later.