Global stock markets have fallen after some members of the US central bank suggested its stimulus measures may be increasing the "risks of future economic and financial imbalances".
The comments came in minutes of the Federal Reserve's last meeting, where the Fed said it had left its monthly $85bn bond-buying plan in place.
US markets fell further on Thursday after recording their biggest drop so far this year on Wednesday.
The Fed comments have raised expectations that the US central bank may scale back its bond-buying programme earlier than predicted.
Currently, the Fed is carrying out its plan of buying $85bn of bonds a month until the US jobs market sees a substantial improvement.
By buying bonds, the Fed keeps interest rates low, which keeps the cost of borrowing for mortgages and other loans low.
However, the minutes of the Fed's meeting in January showed that some members were concerned that the bond-buying programmes could push up inflation or could "foster market behaviour that could undermine financial stability".
The minutes said that "a number of participants" commented that an ongoing review of the effectiveness of the bond programme "might well lead the committee to taper or end its purchases before it judged that a substantial improvement in the outlook for the labour market had occurred".