Antitrust officials in South Korea raided Google Inc's Seoul offices on Tuesday, according to a source familiar with the matter, expanding worldwide regulatory pressure on the Internet giant.

Google, the world's No. 1 search engine, is under investigation by antitrust authorities in the United States and in Europe.

The source, who asked not be identified because of the sensitivity of the matter, said officials from the Korean Fair Trade Commission went into Google's offices on Tuesday morning and planned to return on Wednesday morning.

In April, South Korea's top Internet portals filed a complaint with antitrust regulators alleging that Google was unfairly stifling competition in the mobile search market.

In their joint complaint, NHN Corp and Daum Communications Corp said Android smartphones have Google's search engine installed as a default navigation tool and are systematically designed to make it virtually impossible to switch to another option.

Google said in a statement on Tuesday that it does not require manufacturers of mobile phones that use its Android software to include Google search or other Google applications on the devices.

Google also said it would work with the Korean Fair Trade Commission to address any questions it may have about the company's business.

(Reporting by Diane Bartz and Alexei Oreskovic; Editing by Richard Chang)

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