A human rights group says it has found documents and physical evidence indicating Syrians were arbitrarily detained and tortured in government prisons in the eastern city of Raqqa.
Rebels overran the city in February and facilitated access to Human Rights Watch.
The group says its researchers visited Raqqa in late April and inspected security facilities that belonged to the government and military intelligence.
HRW found documents, prison cells, interrogation rooms and torture devices in the detention centres.
The evidence appears “consistent with the torture former detainees have described”, the New York-based group said in a report.
Raqqa is the first Syrian city under full opposition control.
HRW has been documenting abuses on both sides of Syria’s civil war since the beginning of the conflict in March 2011
Meanwhile, US president Barack Obama and Turkish prime minister Tayyip Erdogan presented a joint front against Syrian dictator Bashar al-Assad even as Mr Obama shies away from deeper US involvement in the conflict, such as sending weapons to the rebels.
The Turkish leader, a key US ally in the Muslim world, said before his trip to Washington that he would press Mr Obama for assertive “further steps” needed to speed Dr Assad’s fall.
Standing alongside Obama in the Rose Garden yesterday as a light rain fell, Mr Erdogan sidestepped a reporter’s question about what would happen if the US did not step up its involvement.
“I’d like to look at things with the glass half-full, instead of half-empty,” Mr Erdogan said.
He and Obama were in “full agreement” on the goals of ending the bloodshed and fostering a representative post-Assad government, he said, speaking through an interpreter.
Mr Obama said he maintains the option of US military measures, particularly because of the broad threat posed by Syrian chemical weapons. Still, he said, there is “no magic formula for dealing with an extraordinarily violent and difficult situation like Syria’s”.
Mr Obama has resisted calls from regional friends, such as Turkey, and from lawmakers such as Republican senator John McCain for the US to send weapons to the rebels and use air power to deny Dr Assad the use of his warplanes and helicopters.