Japan's antitrust watchdog has launched a probe into Google's alleged use of its dominance in online advertising, according to NHK.

The US IT giant is suspected of limiting the operations of rival firm LY Corporation, the operator of mobile messaging-service Line and search-engine Yahoo Japan.

Google has an overwhelming share in Japan's market for what is known as "keyword-targeted" online advertising. This method involves the automatic display of ads based on keywords that users have typed into a search engine.

Sources say Google unilaterally requested Line and Yahoo Japan to stop part of their advertising services during a certain period of time. Since 2010, the group has been receiving technical assistance from Google to offer keyword-targeted ad services.

Sources say the Japan Fair Trade Commission suspects Google's requests violated the antimonopoly law.

They say the company has already submitted a plan that includes preventive measures. If the commission accepts it, Google will likely be exempted from punishments, such as cease-and-desist orders.

American tech giants have been facing more stringent regulations from US and European authorities. Japan's authority is also stepping up its monitoring, including on-site inspections of Japanese subsidiaries.