Foreign investors turned into net sellers of South Korean equities in January, driven by profit-taking following recent gains in the benchmark index, central bank data showed on Thursday.

Offshore investors sold a net US$50 million worth of local stocks last month, reversing from their net purchase of $1.19 billion in December, according to the data from the Bank of Korea (BOK).

Foreign investors, meanwhile, bought $2.44 billion worth of bonds in January, resulting in a combined net inflow of $2.39 billion in local securities, marking the fifth consecutive month of net foreign inflows into domestic securities, reports Yonhap news agency.

"Equity funds posted a net outflow as profit-taking following recent gains in domestic stock prices outweighed expectations of better performances of the semiconductor industry," a BOK official said. "Bond funds saw slower net inflows as investor sentiment weakened amid rising market interest rates."

The benchmark Korea Composite Stock Price Index (KOSPI) jumped more than 22 per cent in January.

The central bank also said daily volatility in the won-dollar exchange rate rose to an average of 6.6 won in January from 5.3 won in December.

"Volatility increased as overseas investment by local investors continued, while the National Pension Service recently reduced its target allocation to overseas equities," the official added.

Meanwhile, South Korean stocks surpassed the 5,500-point mark in midday trading for the first time on Thursday, boosted by sharp gains in blue-chip tech shares, including Samsung Electronics and SK hynix.

The benchmark Korea Composite Stock Price Index (KOSPI) had added 157.13 points, or 2.93 percent, to 5,511.62 as of 10:59 a.m. before dropping just below the threshold as of 11:20 a.m.

This marked the first time the KOSPI surpassed the 5,500-point mark.

Semiconductor shares were the main driving force that pushed up the index, with Samsung Electronics shooting up 6.08 percent and SK hynix soaring 3.26 percent.