Bipolarization in the South Korean corporate debt market continued for five straight months amid no issuance of corporate bonds by small businesses, financial watchdog data showed Sunday.

Large corporations raised 5.56 trillion won ($5 billion) through bond sales in April, while there was no debt issuance by small companies, according to the Financial Supervisory Service.

Corporate bonds issued by big companies were up 28 percent last month from a month earlier, while small firms failed to raise capital through debt sales since December 2012, the watchdog said.

Bonds sold by companies with a credit rating of over "A" accounted for 90.8 percent of the total, indicating firms with low credit were struggling to raise capital in the debt-financing market.

Debt financing among small, low-credit firms has faltered since Kukdong Engineering & Construction, a midsize builder, surprisingly filed for court receivership in September 2012.

Sales of asset-backed securities (ABS) surged to 2.64 trillion won in April from 580 billion won in the prior month as mobile operators rushed to float such bonds with handset installment sale receivables as collateral.

Debts sold by financial institutions increased 6.7 percent from a month earlier to 2.06 trillion won in April, while bank debenture sales declined 11 percent to 1.81 trillion won.

Equity financing market, including rights offering and initial public offering (IPO) had yet to recover to the right track. Rights offering expanded to 456.1 billion won in April from 184.9 billion won in the previous month, but there was only one IPO worth 6.6 billion won.