BEIRUT: Syrian rebels have committed war crimes including torture and killing of detainees, Human Rights Watch said 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.

HRW pointed to the widely reported killing of four men loyal to Assad in the northern city of Aleppo on July 31, the same day they were captured.

It said rebels told HRW they were tried and sentenced to death by a local judicial council. "However it would appear near impossible that the men received a fair trial by a regularly constituted court considering the circumstances and the haste with which they were tried and executed," it said.

Another four people were executed three months ago when rebel fighters stormed a police station in the town of Haffa, in the Mediterranean province of Latakia.

HRW said it was granted access by opposition authorities in several northern towns. Six of 12 people it interviewed in opposition-run detention facilities said their captors "had tortured and mistreated them, in particular by beating them on the soles of their feet".

At least some of the other six detainees were probably also tortured or mistreated despite denying they were beaten, HRW said, citing inconsistencies in their accounts and visible injuries consistent with torture.

The group said it presented its findings to the rebel Military Council for Aleppo Governorate, which responded that it was committed to human rights and would review detention conditions and practices and follow up any violations.

"Declarations by opposition groups that they want to respect human rights are important, but the real test is how opposition forces behave," Houry said. "Those assisting the Syrian opposition have a particular responsibility to condemn abuses."

He called for rights abuses by both sides in the Syria conflict to be referred to the International Criminal Court.

"An ICC referral would give the ICC jurisdiction to investigate crimes committed by both the government and the opposition," Houry said, adding that all United Nations Security Council members including Russia, which has blocked action against Assad, should find it easy to agree if they are concerned about violations in Syria.