An international human rights group says more than 17,000 detainees have died in Syrian government detention facilities since the start of the 2011 uprising against President Bashar Assad.

In a report released on Thursday, Amnesty International estimates that more than 17,723 people have died in custody in Syria between March 2011 and the end of 2015.

It said common methods of torture included forcibly contorting the victim's body into a tyre and flogging on the soles of the feet.

The authorities also used electric shocks, rape and sexual violence, the pulling out of fingernails or toenails, scalding with hot water and cigarette burns.

Syria's conflict has killed more than 250,000 people, displaced half the country's population.