SEOUL, Jan. 3 (Yonhap) -- South Korea's tax agency said Tuesday it will beef up its investigation into large companies and wealthy citizens this year, in a bid to toughen its crackdown on tax evasion and expand the nation's overall tax revenue base.

The National Tax Service (NTS) plans to intensify its crackdown on tax evasion attempts by business groups through unfair trading with their affiliates or subcontractors. It will also expand its investigation into overseas transfers of wealth aimed at avoiding due tax payment.

The measures are part of the tax agency's policy direction for 2012, which was reported to President Lee Myung-bak earlier in the day.

"We will work hard to encourage voluntary and honest tax payments, while making sure the government achieves its targeted tax income by discovering hidden tax sources without burdening the national economy," NTS head Lee Hyun-dong said.

In a related move, the agency said it will establish a system which will verify tax reports filed by high-income earners, such as lawyers, doctors and owners of large private education institutions.

Though major companies and rich citizens will face tougher scrutiny, the NTS said it will exempt smaller enterprises with sales of 10 billion won (US$8.7 million) or less from regular tax audits as long as they are not suspected of tax evasion.

The move is aimed at reducing administrative hurdles for smaller companies which could bear the brunt of worsening economic conditions, it added.

Previously, such tax probe exemptions were granted to companies with sales of 1 billion won or less. Some 400,000 companies are expected to benefit from the latest measures, experts said.