TRIPOLI, Libya: Several suspected diehards of slain Libyan dictator Muammar Gadhafi have been subjected to torture and some have even died in detention centers run by armed militias, human rights groups said on Thursday.

"Several detainees have died after being subjected to torture in Libya in recent weeks and months amid widespread torture and ill-treatment of suspected pro-Gadhafi fighters and loyalists," Amnesty International said.

The London-based watchdog said its delegates met detainees held in Tripoli, in Misrata and in smaller towns such as Ghariyan, who showed visible signs of torture inflicted in recent days and weeks.

"The torture is being carried out by officially recognized military and security entities, as well by a multitude of armed militias operating outside any legal framework," Amnesty International said.

In a separate statement, Doctors Without Borders said it had suspended its work in Misrata, Libya's third largest city, which withstood a devastating siege by Gadhafi's forces during last year's uprising.

"Detainees in the Libyan city of Misrata are being tortured and denied urgent medical care, leading the international medical humanitarian organization Medecins Sans Frontieres (MSF) to suspend its operations in detention centers in Misrata," the group said, referring to itself by its French name.

It said its doctors were increasingly confronted with patients who suffered injuries caused by "torture" during questioning.

"The interrogations were held outside the detention centers," it said.

The concerns raised by the two groups came hours after top UN officials expressed similar fears about Libyan "revolutionary brigades," accusing them of being behind a surge in violence and holding thousands of people in secret detention centers.