As corporate defaults pile up in China’s onshore bond market, a unit of a once-promising energy conglomerate with $4.8 billion of debt and a checkered past said it won’t be able to meet its payment obligation Monday.

CEFC Shanghai International Group Ltd., a unit of the privately-held CEFC China Energy Co.- which takes over Rompetrol in Romania – failed to repay 2 billion yuan ($313 million) of bonds but said it will seek to pay back the notes in six months, according to a statement on the Shanghai Clearing House website. The unit said a week ago it may not be able to repay the notes because its chairman’s failure to “fulfill normal responsibilities” had a huge impact on the company’s operations.

Worries among investors about surging default rates in China are deepening after at least four nonpayments in the onshore market over the past month. Given the magnitude of the CEFC’s debt load, market stability will hinge on how it implements a debt workout plan, according to Fitch Ratings, quoted by Bloomberg.

The firm’s rapid ascent peaked with its agreement in September to buy a $9 billion stake in Russian oil behemoth Rosneft PJSC. That deal fell apart this month as financial troubles emerged after its chairman, Ye Jianming, came under investigation by Chinese authorities and stepped down from management.

The company relied heavily on bond sales for funding over the past two years and its borrowing costs have jumped since Ye was put under investigation, shutting it out from funding in this market.

While creditors to CEFC China Energy have formed a committee to review its asset disposals amid rising pressure to repay debt, Czech financial group J&T Private Investments has decided to take control of the management of the CEFC Europe because of the failure of the parent company to repay debt on time, China’s official Xinhua News Agency reported May 19. CEFC Europe is a Czech-based European branch of the Chinese conglomerate CEFC China Energy.