DOCUMENTS and torture equipment found in Syrian security buildings in rebel-held Raqa show detainees were tortured when President Bashar al-Assad's regime held sway over the city, Human Rights Watch says.

A team of researchers working for HRW toured Raqa in northern Syria in April, a month after the city fell into rebel hands, and found the incriminating evidence, the New York-based watchdog said in a statement.

"The documents, prison cells, interrogation rooms, and torture devices we saw in the government's security facilities are consistent with the torture former detainees have described to us since the beginning of the uprising in Syria," said HRW deputy Middle East director Nadim Houry.

Among the implements the watchdog said it found is a cross-shaped contraption known as "bsat al-reeh" (flying carpet) which "former detainees have said has been used to immobilise and severely stretch or bend limbs".

Torturers used the device to "tie a detainee down to a flat board, sometimes in the shape of a cross, so that he is helpless to defend himself," HRW said, citing former detainees.

"In some cases, former detainees said guards stretched or pulled their limbs or folded the board in half so that their face touched their legs, causing pain and further immobilising them."

HRW researcher Lama Fakih said that although the watchdog has interviewed countless former detainees during the two-year conflict, "being inside the facility makes it so much more real".

"We know people are still being detained and subjected to these practices," she said.

One former detainee told HRW he and his brother were tortured "in turns".

"They started torturing him with electricity for three, four hours, and then they threw him in a solitary cell... They wanted me to tell them who used to go out to demonstrate with me... and they would make me hear my brother's screams," said Ahmed, 24.

Abdullah Khalil, who now heads the opposition civilian council in Raqa, also testified to HRW.

The long-time human rights activist was detained by the security forces on May 1, 2011, less than two months into the uprising.

He was transferred to 17 different security branches while in detention, HRW said.