Trump unveiled a surprise plan last month to hold up to 30,000 migrants at the base -- a facility synonymous with abuses against terror suspects detained there after the September 11, 2001 attacks -- as part of his sweeping crackdown on illegal immigration.

"Arrived at Naval Station Guantanamo Bay on the front lines of the war against America's southern border," Hegseth wrote in a post on X that included a video of him saluting and shaking hands with troops at the base in southeastern Cuba, where he once served as a military officer.

"The protection of the United States' sovereign territory is (the Defense Department's) mission and we've moved swiftly to implement the Commander-in-Chief's E.O.'s on border security," he said, referring to executive orders.

Hegseth had previously written that he would tour base facilities during the visit and "receive briefings on operations supporting illegal immigrant deportations."

A Pentagon announcement on Hegseth's trip said it "underscores the department's commitment to ensuring the security and operational effectiveness" of the base and "provides an opportunity to engage with the dedicated service members and personnel who play a vital role in the base's mission."

-'Unparalleled notoriety' -

The United States began flying migrants to Guantanamo earlier this month, more than 170 of whom were repatriated to Venezuela last week.

US Southern Command said Tuesday there are currently "17 high threat illegal aliens" who arrived at the base over the weekend and are being held there.

The Trump administration has launched what it cast as a major effort to combat illegal migration that has included immigration raids, arrests and deportations on military aircraft, including via Guantanamo.

The American Civil Liberties Union (ACLU) and other advocacy groups said earlier this month that they had filed a suit seeking access to migrants held at the base.

"By hurrying immigrants off to a remote island cut off from lawyers, family, and the rest of the world, the Trump administration is sending its clearest signal yet that the rule of law means nothing to it," Lee Gelernt, deputy director of the ACLU's Immigrants' Rights Project, said at the time.

The Guantanamo prison was opened in the wake of the 9/11 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."

It still holds 15 people incarcerated for militant activity or terrorism-related offenses, among them several accused plotters of the 9/11 attacks, including self-proclaimed mastermind Khalid Sheikh Mohammed.