U.S. Internet giant Google Inc., which has been relatively slow in expanding market presence in South Korea, was fast catching up to local rivals in advertisement sales last year, industry sources said Thursday.

According to the sources who have access to internal data, Google raked in some 100 billion won ($93.8 million) in revenue from its online display adverts in South Korea last year, a significant improvement from the 40 billion won posted a year earlier.

Google's robust performance raised  alarm in the South Korean portal market which local players have long dominated.

Market watchers said Google, which only takes up around 10 percent of the local portal market, may eventually emerge as a threat to home players. 

Display advert revenue for Naver Corp., the top industry player with a 70 percent share of the market, edged down 7 percent over the cited period to 323.5 billion won.

Daum Communications Corp., the operator of South Korea's second most-visited portal, also remained sluggish in the sector by only edging up 0.4 percent on-year to 230.5 billion won.

The size of South Korea's online display advertisement market grew 7 percent over the cited period to reach $644 billion last year.

"Google's improved performance came as Psy's 'Gangnam Style'

received huge attention on YouTube," a market watcher said. Google currently handles advertisements on the video-sharing website. The South Korean rapper's 2012 global hit song has garnered some 1.9 billion views as of Thursday. 

The combined size of South Korea's online advert market came to

2.4 trillion won in 2013, up 16 percent from the previous year, with search ads accounting for 54 percent, trailed by display ads with 26 percent and mobile ads with 19 percent, according to the Korea Onlinead Association. (Yonhap)