Some 20 wealthy people, mostly conglomerate owners, have been under investigation by the country's financial watchdog for their suspected illegal foreign exchange transactions, industry sources said Monday.

According to the sources, they had received a combined $50 million in their bank accounts between 2011 and 2014, which they claimed were investment returns, proceeds from asset sales made overseas, and wage income.

But some of them had not reported such cases to the concerned authorities before, the sources said.

In South Korea, residents are required to report a certain amount of their cross-border foreign exchange transactions to related agencies as the country cracks down on tax evaders and those who sneak their wealth out of the country.

"We are looking into their foreign exchange transactions that are suspected of violating rules," said an official with the Financial Supervisory Service, the country's financial watchdog. "But we can't provide any details on the case."

The watchdog is parsing how they had created the wealth and whether they observed related requirements on foreign exchange transactions.

Last year, several high-profile local businessmen were found to have set up paper companies in popular tax havens in an apparent bid to stash away funds or evade taxes.

In response, the country's customs office and financial authorities have been stepping up measures to prevent offshore tax evasion and overseas concealment of wealth. (Yonhap)