Noble has suffered fresh setbacks this month, reporting a quarterly loss that pummelled its stocks and bonds. Citing sources, Reuters reported on Monday that China's state-owned Sinochem is no longer pursuing an investment in Noble due to concerns over its finances and business outlook.

This has deepened its struggles to repair investor confidence in the past two years after a questioning of its accounts by Iceberg Research and a commodities downturn that triggered a share price collapse, credit downgrades and a series of writedowns, asset sales and fund raising. Noble has stood by its accounts.

"We downgraded Noble because we believe the company's capital structure is not sustainable," S&P said late on Monday, cutting Noble's long-term corporate credit rating to CCC+ from B+.

Noble shares dropped to as low as S$0.45.

(Reporting by Miyoung Kim; Editing by Edwina Gibbs)