The probe by the South's Fair Trade Commission (FTC) is focused on whether Google is unfairly forcing smartphone makers to use its Android operating system, Yonhap news agency said.
Officials from the FTC visited the firm's Seoul office last week for an "on-site inspection," Yonhap added.
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The FTC declined to confirm or deny the report, while no Google spokesperson was immediately available to comment.
The reported probe came after the EU filed new anti-trust charges on July 14 against Google's advertising business, accusing it of restricting some websites from displaying ads from Google's competitors.
It is one of three EU cases now pending against Google, including charges filed in April over the company's Android operating system.
The FTC had filed charges five years ago on complaints by Seoul search engine operators that Google was forcing handset makers to preload Google's search engine on their Android-powered mobiles.
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But in 2013, the FTC concluded that Google's share of the local online search engine market was too small to threaten fair competition or limit consumer choice.
Google enjoys only 10 percent of a market that is dominated by Korean search engines Naver and Daum, but its Android platform has a far bigger presence in the smartphone sector.
Almost all the smartphones and tablet computers produced by local electronics giants like Samsung and LG are powered by Android.
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