SHARES of Noble Group tumbled a further 13 per cent on Friday, a day after the Singapore Exchange asked the commodities group to explain the trading in its shares after they fell by as much as 12 per cent.

At 10:09am, Noble was trading around S$0.455 a share, down S$0.065, or 12.50 per cent. More than 78 million shares changed hands.

According to Bloomberg, Noble has used S$131 million on its share buyback recently. The only time Noble spent more than that on an asset since 2011 was when it paid US$140 million for a Jamaican alumina plant in 2014.

Noble's stock has lost about half its value since mid- February, when Iceberg Research published criticism of the firm's accounting.

Noble has rejected the criticism and hired PricewaterhouseCoopers this month to review its practices.