BENGHAZI, Libya (Reuters) - Saif al-Islam, a son of the late Libyan leader Muammar Gaddafi, remains in the prison in western Libya where he has been held since the 2011 uprising that toppled his father, a military source said on Thursday, contradicting media reports that Saif had been released.

Since his father's downfall, Saif has been held in Zintan, a mountainous western region, by one of the factions that began contending for power after Gaddafi was killed in 2011 and have now split the country into warring fiefdoms.

"We deny that Saif Islam has been released," the Zintan military source told Reuters.

Last July, a Libyan court sentenced Saif to death in absentia for war crimes, including killing protesters during the 2011 revolt.

Zintani forces refused to hand Saif over to any other authorities, saying they did not trust Tripoli to guarantee he does not escape. But they agreed to let him be tried there, though he appeared mainly by video link.

Libya's new U.N.-backed unity government, set up in Tripoli three months ago, is trying to bring together the various factions struggling for control of Libya and its oil resources.

Brigades of former rebels backing rival political factions remain the main power brokers in Libya, where the unity government has struggled to exert its influence. Islamic State has also gained ground in the chaos.

(Reporting by Ahmed Elumami, writing by Patrick Markey, editing by Larry King)