The NCUA is suing a total of nine institutions over the sale of nearly $2.4 billion in mortgage securities to Southwest and Members United corporate credit unions, which it said subsequently collapsed.
NCUA board chairman Debbie Matz said: "We continue to pursue accountability and recovery in the wake of billions of dollars in sales of faulty securities that led to the collapse of several corporate credit unions and handed the industry the costly bill of paying for the losses,"
The NCUA lawsuits filed in the Manhattan district court said the banks, including part-nationalised RBS, made misrepresentations in connection with the underwriting and sale of the mortgage securities.
In the claim filed against RBS, the NCUA accuses the bank of providing "untrue statements of material fact and/or omissions of material facts".
It also claimed the bank did not stick to its underwriting guidelines. This, the body said, meant they were riskier than anticipated.
RBS is already being investigated, alongside HSBC and Barclays, by the Federal Housing Finance Agency, the US housing regulator, over the selling of mortgage bonds.