TOKYO -- Japanese investment bank Nomura on Friday reported a 63.5% drop in pretax profit for the fiscal first half that ended last month, after booking a charge of 39 billion yen ($343 million) to provide for a possible loss from a lawsuit over a U.S. trade.

The trade took place more than a decade ago before the Lehman crisis and is unrelated to it, Takumi Kitamura, Nomura's chief financial officer, told a news conference.

Kitamura declined to provide further details about the provision. "It is very regrettable that we had to book such a loss, even if it was over a trade that took place a long time ago," he said. "It is difficult to eliminate every risk from the overseas business."

He added that Nomura will work to ensure that such an incident does not recur in even 5 or 10 years.

For the April-September first-half, pretax profit fell to 97 billion yen from 265 billion yen in the same period last year. Revenue dropped 16% to 784 billion yen.

The results reflect turbulence in U.S. financial markets as investors fretted during the period about a possible monetary tightening by the U.S. Federal Reserve, Kitamura said. For a broker like Nomura with a focus on fixed-income products, "it was a difficult market environment," he said.

By contrast, investment banking displayed a strong performance for a fourth straight quarter, as fees from M&A advisory and equity issuance jumped in Japan and abroad.

Nomura's earnings setback follows a $2.87 billion loss from its trade with U.S. family office Archegos Capital Management in March and illustrates its challenge in managing risks in its U.S. business.

The investment bank's U.S. business comprises trading and investment banking. It was the main profit driver last year but also proved a source of earnings volatility, presenting a challenge to CEO Kentaro Okuda on how to achieve more stable, if less stellar, earnings.

Nomura announced on Friday that it has established a board-level risk committee to further strengthen its global risk management. The committee is led by an outside director and is tasked with formulating a risk management framework. Nomura also announced an increase in the number of risk managers across divisions.