The company logo of Noble Group is displayed at its office in Hong Kong, China January 22, 2016. REUTERS/Bobby Yip/File Photo

SINGAPORE (Reuters) - Shares in Noble Group NOBG.SI tumbled 17 percent on Friday, hit by a critical report from Iceberg Research that reiterated its concerns about the commodity firm's accounting practices.

Iceberg issued its report after sources told Reuters this month that China’s state-owned Sinochem was in early talks to buy an equity stake in Noble. Noble has since confirmed it was in discussion about a possible strategic investment in the firm.[nL4N1FY1ZJ]

A Sinochem source said the company was still conducting due diligence on Noble, which typically takes six months to a year.

Iceberg said in its report that Noble was not worth its book value.

Noble declined to comment on Friday’s report except to say it was not aware of any reason for its stock price slide other than the Iceberg article.

It has previously rejected Iceberg’s claims, first made in February 2015, over its accounting practices. As investor confidence in the company slumped, Noble sold off assets and completed a rights issue to raise some $2 billion as well as restructured its operations.

The 17 percent drop in Noble’s shares has wiped out most of the gains made since Feb. 14 on hopes of a deal with Sinochem.