WASHINGTON (REUTERS) - The United States will transfer four detainees to Saudi Arabia from the Guantanamo prison in the next 24 hours, a US official said on Wednesday (Jan 4), in President Barack Obama's final push to shrink the inmate population despite pressure from President-elect Donald Trump to halt such releases.

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

If the final transfers go according to plan, about 40 prisoners will remain at Guantanamo, marking Mr Obama's failure deliver on his long-time pledge to close the controversial facility at the US naval base in Cuba.

Mr Trump has vowed to keep the Guantanamo military prison open and "load it up with some bad dudes."

The Obama administration last month notified Congress of its intention to make the additional transfers, Reuters reported earlier. They make up the bulk of the 23 prisoners who have been declared in parole-style hearings to be safe for repatriation or resettlement in other countries.

Mr Trump on Tuesday signaled his view that all of those held at Guantanamo should remain, despite 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 Mr Trump's objections and said transfers from Guantanamo, opened by President George W. Bush to hold terrorism suspects rounded up overseas following the Sept 11, 2001 attacks, would continue until Mr Trump takes office.

The nationalities of the four prisoners being transferred were not immediately known.