Commerzbank AG (CBK.XE) said Wednesday it isn't planning a firesale of its shipping credit portfolio, which amounts to around 18 billion euros ($23.6 billion).

"We want to sell our loans over time and preserve value; there won't be any firesales, which includes shipping credit," Stefan Otto, who is responsible for non-core assets at Commerzbank's Deutsche Schiffsbank AG unit, said in a statement.

The bank was responding to market speculation that it is struggling to sell the portfolio, and that bids are so low that the necessary writedowns related to a sale would jeopardize the bank's equity. The share price tumbled on the speculation. At 1130 GMT, Commerzbank was trading down 3.1% at EUR6.83.

Deutsche Schiffsbank was once one of the world's largest shipping financiers. In November 2012, Commerzbank said it was targeting a 40% reduction in shipping loans by 2016.

According to a report in Boersen-Zeitung earlier Wednesday that cited sources close to the company, Commerzbank is in final negotiations with France's BNP Paribas over the sale of the German bank's custody services unit. The deal will be signed by mid-July, according to the report. Commerzbank declined to comment on the report. The unit, which has custody of EUR92 billion of assets, could be valued at over EUR200 million.