US Defence Secretary Robert Gates says that meeting the deadline of 22 January set by President Barack Obama for closing the Guantanamo Bay camp will be "tough".
In interviews with US TV networks on Sunday, Mr Gates said that closing the camp was proving more complicated than expected.
The BBC reports he played down the importance of possibly pushing the date back.
More than 220 inmates are in the camp. Some are expected to be sent to other countries, others could face military tribunals or be tried in courts in the United States.