Facing tightening credit, U.S. banks have quietly borrowed more than $50 billion over two months from the Federal Reserve using a little known and newly created credit facility, according to a report.

The banks are using the Fed's Term Auction Facility, which was announced in late december, to borrow at relatively attractive rates against a broader range of collateral than they would normally be allowed to do, according to the Financial Times.

The Fed doesn't report which banks borrow.

However the report states that analysts are growing concerned about the stress developing in opaque corners of the US banking system and increasing reliance on indirect government support.