"No Guantanamo detainees have been vaccinated," Pentagon spokesman John Kirby tweeted. "We're pausing the plan to move forward, as we review force protection protocols. We remain committed to our obligations to keep our troops safe."

The Navy base at Guantanamo Bay, Cuba houses detainees in the US "war on terror," including top Al-Qaeda figure and alleged 9/11 attack planner Khalid Sheikh Mohammed.

The Department of Defense told US media earlier in the week that it would offer vaccines to its detainees and prisoners, to be administered "on a voluntary basis."

Backlash came from Republican lawmakers including the party's top congressman, Kevin McCarthy.

"President Biden told us he would have a plan to defeat the virus on day 1. He just never told us that it would be to give the vaccine to terrorists before most Americans," he said on Twitter.

Added New York congresswoman Elise Stefanik: "It is inexcusable and un-American that President Biden is choosing to prioritize vaccinations for convicted terrorists in Gitmo over vulnerable American seniors or veterans."

The United States has been the world's hardest-hit country by the coronavirus pandemic, in absolute terms, with 436,000 deaths and nearly 26 million cases.

President Joe Biden has pledged to vaccine 100 million Americans in his first 100 days in office, but so far the country's mass inoculation drive has been beset by stumbles, including a shortfall in vaccines and widespread technical difficulties for eligible Americans trying to make appointments.

According to health officials, the US has so far administered just under 30 million of the nearly 50 million doses distributed in the country.