Dozens of US and international human rights organizations are decrying the Trump administration’s plans to establish a migrant “camp” for fleeing Cubans at the Guantánamo Bay military base if the island nation’s crisis worsens under pressure from the US, according to a letter to members of Congress on Friday.

The 85 groups plan to submit the joint letter, exclusively shared with the Guardian, to US senators and House representatives, expressing their “profound concern” with comments made last month by a top Department of Defense commander, and describing any prospect of further migrant detention at the base as “deeply troubling and unacceptable”.

The commander told Congress last month that in the event of what one Republican senator described as “any humanitarian crisis” in Cuba, the Pentagon would “set up a camp” at the Guantánamo Bay US base to “deal with” migrants.

The United Nations already warned in February that Cuba could be on the brink of a humanitarian “collapse” following Donald Trump’s attempts to block oil supplies from reaching the island and the US president repeatedly threatening Cuba’s communist government.

“Guantánamo should be a relic of the past,” the organizations’ letter to the lawmakers reads. “We call on you to act without delay to ensure not another dollar goes towards the detention facilities at Guantánamo, ensure the base is never again used for unlawful mass detention of any group of people, and end the coercive and punitive policy of sanctions and the embargo driving the humanitarian crisis.”

The letter has been signed by groups including the Center for Constitutional Rights, which has represented dozens of detainees held at Guantánamo during America’s so-called war on terror since the 2001 terrorist attacks on the US on September 11. The less well-known migrant detention facilities are separate from the terrorism-related detention center at the base.

“Both have long been known for inhumane conditions, mistreatment, and due process violations,” Friday’s letter states.

Other signatories include a wide variety of US and international advocacy, religious, legal and policy groups such as the Center for Victims of Torture, a chapter of Amnesty International, the Center for Economic and Policy Research, Refugees International, the justice team at Sisters of Mercy of the Americas, the Council on American-Islamic Relations, Detention Watch Network and more.

“Guantánamo stands alone in its reputation as the most heinous and dangerous place the US president can possibly send you – outside view, under the radar, beyond the law,” said Yumna Rizvi, a senior policy analyst for the Center for Victims of Torture. “The US can provide immediate relief to the Cuban people, but instead is choosing to cause irreparable harm and then double down by threatening detention at Guantánamo for Cubans who need to flee because of his own actions. It is outrageous.”

During a US Senate armed services committee hearing on 19 March, Gen Francis Donovan, the commander of US Southern Command (Southcom), which oversees the Guantánamo Bay base on a tiny portion of US-claimed land on Cuba, testified it could be used to establish a new version of a migrant “camp” for Cubans.

When asked by the Republican senator Tom Cotton of Arkansas about military preparations in response to any humanitarian crisis in Cuba, Donovan said that Southcom was under orders to “support DHS [the Department of Homeland Security] in a mass migration event”.

“We would follow either at sea, or primarily the shore, primarily at Guantánamo Bay, to set up a camp to deal with those migrants or any overflow from any situation in Cuba itself,” Donovan said.

In a radical escalation of immigration enforcement early last year, Trump signed an executive order to expand migrant detention operations at Guantánamo and began sending immigrants from the US to be held there, often en route to deportation. About 780 immigrants detained in the US have been sent to Guantánamo since last February, sometimes for weeks at a time, according to a tally by the New York Times.

The Migrant Operations Center (MOC) is a facility at Guantánamo under the purview of both the Pentagon and the DHS and has been used for decades to detain a small number of migrants interdicted at sea.

The Guardian revealed last year that the private US government contractor running the MOC has fallen under scrutiny for conditions inside other immigration jails it operates. In addition to using the MOC, the administration began to quickly put up tents in a costly and controversial plan to detain an estimated 30,000 migrants at Guantánamo amid the second Trump administration’s focus on a harsh anti-immigration agenda. But the tents were never used and the installations were dismantled.

After the January abduction of the Venezuelan president, Nicolás Maduro, the Trump administration turned its attention on Cuba. “Cuba is next, by the way,” Trump said in March.

“If the Trump administration is worried about Cuban migration, the solution is simple: stop intentionally impoverishing the Cuban people through an embargo and fuel blockade,” said Michael Galant, a senior research and outreach associate with the Center for Economic and Policy Research, one of the groups that signed on to Friday’s letter.