The Pentagon is preparing to transfer six detainees from Guantanamo Bay to Uruguay in the next 30 days, according to U.S. officials, part of an effort to further reduce the prison’s population.

Controversy over the decision to swap Sgt. Bowe Bergdahl for five Taliban detainees held in Guantanamo has raised questions about whether Congress will try to block further moves by the Obama administration to try and close the prison.

The Pentagon has notified Congress that it intends to transfer the detainees, which include four Syrians, a Palestinian and a Tunisian. The notification of Congress was first reported Wednesday by the New York Times.

An official said the men being sent to Uruguay were low-level detainees, who all have been approved for transfer years ago.

“This is just another transfer,” said the official.

Since Mr. Obama announced a renewed effort to close Guantanamo more than a year ago, the Pentagon has stepped up the pace of transfers. Congress typically has 30 days to object to any transfer and move to block them.

While lawmakers haven’t moved to block routine transfers in the past, many in Congress are angry at the Obama administration for failing to notify them about plans to transfer the five senior Taliban detainees to Qatar.

There are 149 detainees in Guantanamo, including the six the Pentagon wants to send to Uruguay.

The Uruguay deal came after the country’s president, José Mujica, agreed in May to take the detainees, provided they were allowed to live freely in his country.

There are 78 detainees in Guantanamo who have been approved for transfer. Another 71 haven’t been approved for transfer.

Before the transfer of the five Taliban prisoners to Qatar, the Senate Armed Services Committee approved a version of the defense policy bill loosening restrictions on transferring detainees from Guantanamo. The House version of the defense appropriation bill, approved in the wake of the swap for Sgt. Bergdahl, would block the transfer of all detainees from Guantanamo.

Write to Julian E. Barnes at julian.barnes@wsj.com