WASHINGTON — Cuba temporarily detained 178 political prisoners in January, Tom Malinowski, the State Department’s senior human rights official, said Tuesday.

That was a decline from December, when there were 489 such detentions. The average monthly figure for 2014 was 741, added Mr. Malinowski, who based that number on reports by the Cuban Commission for Human Rights and National Reconciliation, an independent human rights group.

Still, Mr. Malinowski, appearing before the Senate Foreign Relations Committee, cautioned that it was too soon to say if the reduced number of detentions represented a trend.

“The nature of the Cuban regime has not changed, and we have not claimed so,” Mr. Malinowski said.

The Obama administration’s decision to restore diplomatic relations with Cuba was the major focus of a hearing led by Senator Marco Rubio, the Florida Republican and potential presidential candidate who has been highly critical of White House Cuba policy.