By Guy Faulconbridge LONDON (Reuters) - A Syrian military police photographer has supplied "clear evidence" showing the systematic torture and killing of about 11,000 detainees in circumstances that evoked Nazi death camps, former war crimes prosecutors said. The trove of harrowing photographs ratchets up the pressure on Syrian President Bashar al-Assad, who the United States and its Western allies say has committed war crimes against his own people during the civil war. Assad, once courted by Western leaders and now supported by Russia and Iran, has denied war crimes, saying he is fighting "terrorists" who want to use Syria to sow chaos across the Middle East. "There is clear evidence, capable of being believed by a tribunal of fact in a court of law, of systematic torture and killing of detained persons by the agents of the Syrian government," the three prosecutors said in the 31-page report.