President Donald Trump last week ordered the preparation of a 30,000-person "migrant facility" at the base, which is primarily known as a detention center for suspects accused of terrorism-related offenses, but which also has a history of being used to hold migrants.

"These 10 high-threat individuals are currently being housed in vacant detention facilities," the Pentagon said in a statement.

"US Immigration and Customs Enforcement is taking this measure to ensure the safe and secure detention of these individuals until they can be transported to their country of origin or other appropriate destination," it said.

Officials said Tuesday that flights to the base had started, as part of what the Trump administration is casting as a major effort to combat illegal migration that has also included immigration raids, arrests and deportations on military aircraft.

The president has made the issue a priority on the international stage as well, threatening Colombia with sanctions and massive tariffs for turning back two planeloads of deportees.

The Guantanamo prison was opened in the wake of the September 11, 2001 attacks and has been used to indefinitely hold detainees seized during the wars and other operations that followed.

Conditions there have prompted outcry from rights groups, and UN experts have condemned it as a site of "unparalleled notoriety."