Five former employees of SMBC Nikko Securities have pleaded not guilty at their first court appearance over alleged illegal trading and market manipulation.

Prosecutors indicted six former employees, including senior officials, and the company for making illicit trades on certain stocks to prevent price falls.

Five of them appeared in Tokyo District Court on Wednesday. They include the firm's former Deputy President Sato Toshihiro and former equity department chief Yamada Makoto.

Prosecutors told the court that the suspected irregularities occurred when Yamada and others engaged in transactions outside of trading hours known as "block offers."

They are alleged to have illegally bought a large volume of stocks to prevent a price collapse and informed Sato and others of the purchases.

Yamada's attorney affirmed that they bought the stocks, but denied any illegality, saying that the purchases were not made in an attempt to stabilize the market price.

In February, final court rulings were handed down on the company and the former deputy head of the equity division. The firm was fined 700 million yen and additional charges of 4.47 billion yen, or a total of about 37 million dollars. The official was sentenced to 18 months in prison suspended for three years.