US President-elect Donald Trump says there must be no further releases of detainees from the Guantanamo Bay detention centre in Cuba.
He said those left were "extremely dangerous people and should not be allowed back onto the battlefield".
President Barack Obama had vowed to close the jail during his tenure and has transferred out many prisoners.
Around 60 inmates remain and the White House said later on Tuesday it expected more transfers before 20 January.
Mr Trump had opposed Mr Obama's closure plan during the presidential election campaign.
Last February he said: "This morning, I watched President Obama talking about Gitmo, right, Guantanamo Bay, which by the way, which by the way, we are keeping open.
"Which we are keeping open... and we're gonna load it up with some bad dudes, believe me, we're gonna load it up."
Analysis - Gordon Corera, BBC security correspondent
President Obama promised to close Guantanamo when he took office. But what will Trump do?
His latest tweet suggests he will halt one aspect of the Obama policy - the attempt to reduce numbers by sending abroad those cleared for release.
But that is not quite the same as the president-elect following up one of his promises on the campaign trail to actually resume bringing new detainees in to Guantanamo and increasing numbers. That would be a much more radical step.
So would a related issue of returning to practices such as waterboarding detainees which was talked about during the campaign but where the language has been less clear since election night.
With Guantanamo as with so many other issues, no one is quite sure what to expect after 20 January.
On 17 December, US media reported that the Pentagon was planning to transfer another 17 inmates in the coming weeks.
Guantanamo Bay is being used to detain what Washington calls "enemy combatants", following the 11 September 2001 attacks in the US.
The first 20 detainees were transferred to the facility on 11 January 2002, and a total of 780 men have been held since then - the vast majority without charge or criminal trial.
Mr Obama has approved transfers regularly but the Republican-dominated Congress tightened restrictions, requiring the Pentagon to report that security standards have been met at least 30 days before any such move.
Those removed from Guantanamo have been transferred to other countries and lawmakers have resisted any attempts to allow prisoners into the US.