The US government has hit the Japanese investment banking unit of the Royal Bank of Scotland with a $50m criminal fine after it pleaded guilty last year to wire fraud over its role in the Libor fixing scandal.

RBS Securities Japan said in court documents that, from at least 2006 to 2010, around 20 of its traders sought to manipulate Libor in a direction that benefited their trading positions.

The unit also confirmed that it "accepts responsibility" for its employees' misconduct.

"[This] sentencing of RBS is an important reminder of the significant consequences facing banks that deliberately manipulate financial benchmark rates," said Mythili Raman, acting head of the US Department of Justice's criminal division, in a statement.

US District Judge Michael Shea in Hartford, Connecticut has approved the fine and plea.

In April 2013, Japan's Financial Services Agency (FSA) ordered RBS's Tokyo-based investment bank to overhaul its compliance procedures, after the bank pleaded guilty to criminal charges. A number of employees were found to have sought and colluded to manipulate interbank lending rates.

The FSA ordered RBS Securities Japan to improve its compliance functions, which includes "clarifying the responsibility of the management and staff regarding [rate manipulation] and take preventive measures against recurrence of [Libor rigging]."

The FSA has also forced the unit to submit a report to the regulator by May this year, and every three months thereafter, in order to prove it is planning and implementing a change to its compliance procedures.

In February 2013, RBS agreed to pay £390m ($612m, €451m) to settle US and UK charges related to the manipulation of the benchmark lending rate known as Libor, as well as pleading guilty to a criminal charge of wire fraud from a Japanese subsidiary.

When RBS Securities Japan pleaded guilty to one criminal charge of wire fraud, the bank entered what's called a deferred prosecution agreement under which it cooperates with the US Department of Justice, pay a $150m fine and have that charge and another antitrust charge deferred.

RBS was also ordered to pay an additional £87.5m to the UK's Financial Services Authority.

The case is U.S. v. RBS Securities Japan Ltd, U.S. District Court, District of Connecticut, No. 13-cr-00073.