By Matt Spetalnick

WASHINGTON, Dec 20 (Reuters) - President Barack Obama plans to transfer as many as 18 more prisoners from the Guantanamo Bay military prison before leaving office, a source close to the matter said, further shrinking the inmate population but still far short of meeting his longtime pledge to close the facility.

The Obama administration notified Congress it intends to send the detainees, nearly a third of the remaining 59 held at the U.S. naval base in Cuba, to at dong trung ha thao least four countries, including Italy, Oman, Saudi Arabia and the United Arab Emirates, before President-elect Donald Trump is sworn in on Jan. 20, the source said.

The notification came just before this week's deadline, which by law requires Obama to give Congress 30 days' warning before moving prisoners out of Guantanamo. It will be the last in a flurry of recent transfers aimed at leaving as few inmates as possible for the next administration.

But the transfer plan - first reported by the New York Times - also signifies that despite Obama's pledge dating back to the 2008 presidential campaign to close the facility, it is all but certain to be turned over to Trump. He has vowed to keep it open and "load it up with some bad dudes."

The administration wants to move out 17 or 18 of the 22 prisoners who have been declared eligible for transfer in parole-style hearings, the source said, while cautioning that it was still possible one or more of the countries could back out.

If the transfers go according to plan, 41 or 42 prisoners would be left at Guantanamo, including 10 alleged plotters of the Sept. 11, 2001 attacks charged in military tribunals. The others have not been charged but are deemed too dangerous to release.

The White House and Pentagon declined to comment.

Obama, who inherited 242 detainees when he took office and has called it a "recruiting tool" for terrorists, has slowly whittled the number down to the lowest since shortly after his predecessor George W. Bush opened the facility to hold terrorism suspects rounded up overseas following the Sept. 11 attacks.

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.