BEIRUT - Syrian rebels have committed war crimes including torture and killing of detainees, Human Rights Watch said on Monday, calling on countries which support those fighting Bashar Assad's rule to press them to respect humanitarian law.

The New York-based group said it had documented more than 12 cases where rebel fighters had killed their captured opponents, while at least six detainees interviewed by HRW said they had been tortured and mistreated.

"Time and again Syria's opposition has told us that it is fighting against the government because of its abhorrent human rights violations," HRW's deputy Middle East director Nadim Houry said. "Now is the time for the opposition to show that they really mean what they say."

Rights groups have accused Assad's forces of carrying out massacres, summary executions and widespread torture in detention since the outbreak of Syria's uprising 18 months ago.

But video footage of rebel fighters hurling bodies off high buildings and gloating last week over the corpses of 20 uniformed men, their hands bound behind their back, suggest brutality among rebel ranks as well.

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