The Federal Reserve just bought $43.6 billion in US treasuries in the span of a week, sparking concerns that a quiet quantitative easing operation is underway.

New filings show the Fed purchased $8.8 billion in 30-year bonds on May 8th via its System Open Market Account (SOMA) - a move that followed a $34.8 billion purchase earlier that same week.

The move has triggered allegations that "stealth QE" has arrived, with a MarketWatch Op-Ed from Charlie Garcia calling the move "monetary policy on tiptoes."

The Fed denies the claims, and has long stated such purchases are routine reinvestments of maturing securities to adjust the money supply and influence interest rates to meet its targets.

The Fed's buying spree follows a major Treasury sell-off from China.

New numbers from the Treasury Department show China sold $18.9 billion in US bonds in March, while most other countries increased their holdings.

China now holds $765.4 billion in US Treasuries and is in third place behind the UK and Japan, which hold $779 billion and $1.13 trillion, respectively.