Since the rebel forces?captured the capital Tripoli,?over 10,000 prisoners arrested by Muammar Gaddafi's government have been released, a rebel military spokesman said on Sunday.

About 50,000 prisoners were still missing, and it would be "catastrophic" if they had already been killed after being arrested, the rebel military spokesman Col. Ahmed Bani?said at?? at a press conference in the rebel bastion of Benghazi.

"The number of people arrested over the past months is estimated at between 57,000 and 60,000," Bani said.

Expressing deep concern about their fate, he said "many people in Tripoli are now discovering mass graves around former detention centres and the prison in Abu Slim."?

Head of the Libyan National Transitional Council (NTC) Mostafa Abdel Jalil Saturday vowed fair trials for those who once worked with Gaddafi, and said the NTC will raise the reward for those killing or capturing Gaddafi.

On the battle front, the rebel forces were closing in on Gaddafi's hometown of Sirte, one of the last few areas still held by troops loyal to the toppled Libyan leader, from the west and east.

Rebel leaders vowed to take Sirte by force if their negotiations with the region's tribal leaders fail.