Shares of commodity trader Noble Group sank to a 13-year low yesterday, prompting a query from the Singapore Exchange (SGX).

The stock tumbled 17.9 per cent or 2.9 cents to 13.3 cents at the close - its lowest level since May 2003.

Noble shares were trading at 13.8 cents - down 2.4 cents - at around 2.35pm when SGX highlighted the "unusual price movements".

The Hong Kong-based Noble responded that it was unaware of the reason for the trading movements.

Noble raised about US$500 million (S$670 million) in a recent one-for-one rights issue to cut debt and boost liquidity. More than 6.5 billion rights shares will be issued.

Its shares traded at 30 cents each on June 2 - the last trading day before the exercise was announced. The counter has since lost more than half of that value as it tumbled to 13.3 cents yesterday.

Noble said the issue - in which each rights share was priced at 11 cents - was oversubscribed, and the new shares are expected to start trading tomorrow.

The rights issue was supported by Noble executive chairman Richard Elman as well as sovereign wealth fund China Investment Corp's unit Best Investment Corp.

Noble's shares have slipped more than 60 per cent in the past 12 months amid poorer earnings and the loss of its investment-grade rating. The junk-graded commodity trader booked a 62 per cent drop in its first-quarter net profit in the three months to March 31 to US$40.5 million, while revenue fell 32 per cent from a year earlier to US$11.39 billion.

In February, Noble reported its first full-year net loss in almost two decades of US$1.67 billion - a far cry from its US$132 million net profit in 2014

Noble has had a tumultuous year, having to battle weak markets and the fallout from questions over its accounting methods.

It was accused by Iceberg Research in February last year of hiding losses and overstating assets through the use of accounting loopholes - a claim Noble rejected.