The United States will transfer four detainees to Saudi Arabia from the Guantanamo Bay military prison in the next 24 hours, U.S. officials said on Wednesday, in President Barack Obama's final push to shrink the inmate population there despite pressure from the president-elect to halt such releases.

It will be the first in Obama's final flurry of transfers aimed at sending as many as 19 prisoners to at least four countries, including Italy, Oman and the United Arab Emirates, before Donald Trump is sworn in on Jan. 20.

If the final transfers go according to plan, only about 40 prisoners will remain at Guantanamo, despite Obama's pledge to close the controversial facility at the U.S. naval base in Cuba.

The Republican president-elect has vowed, however, to keep the Guantanamo military prison open and "load it up with some bad dudes." The Obama administration notified Congress last month of its intention to make the additional transfers.

Those being transferred make up the bulk of the 23 prisoners declared in parole-style hearings to be safe for repatriation or resettlement in other countries.

Trump on Tuesday signaled his view that all of those held at Guantanamo should remain, despite lengthy inter-agency reviews that deemed many of them eligible to leave.

"There should be no further releases from Gitmo," he tweeted. "These are extremely dangerous people and should not be allowed back onto the battlefield."

The White House dismissed Trump's objections and said transfers from Guantanamo, opened by former President George W. Bush to hold terrorism suspects rounded up overseas following the Sept. 11, 2001 attacks, would continue until Trump takes office.

"We expect that there will be more transfers," said White House spokeswoman Emily Horne. "We do not comment on the progress of individual detainee transfers until they are completed."

The two U.S. officials, who spoke on condition of anonymity, were unable to provide the nationalities of the prisoners who were headed to Saudi Arabia. Last April, the desert kingdom - one of the top U.S. allies in the region - accepted nine Yemenis under a long-negotiated deal between Washington and Riyadh.

Of the 59 prisoners remaining at Guantanamo before the latest transfer, 10 face charges in military commissions, including alleged plotters of the Sept. 11 attacks, while about two dozen have not been charged but have been deemed too dangerous to release.

Obama, who inherited 242 detainees at Guantanamo Bay when he took office in 2009 and has called it a "recruiting tool" for terrorists, has slowly whittled the number downward.

Under Bush, the prison came to symbolize aggressive detention practices that opened the United States to accusations of torture. Obama's efforts to close the prison have been blocked by mostly Republican opposition in Congress, which has barred him from moving any prisoners to the U.S. mainland.

Foot-dragging by Pentagon officials has also been blamed for slowing transfers. Administration officials have also made clear that Obama has no intention of resorting to the legally risky option of using executive action to close the prison before leaving office.