Paulo Pinheiro, the U.N. inquiry's lead investigator, said his team collected "a formidable and extraordinary body of evidence" that government forces have increased attacks against citizens, The New York Times reported Monday.
In a report presented to the U.N. Human Rights Council in Geneva, Switzerland, Pinheiro said investigators found evidence that both government and anti-government forces committed war crimes and crimes against humanity, noting that the evidence collected included names and identified units.
He recommended that the evidence should be forwarded to the U.N. Security Council, which could refer the abuses to the International Criminal Court.
The stepped-up violence was marked by a growing presence of "foreign elements," including religious militants, Pinheiro said.
Pinheiro said opposition group Free Syrian Army appeared to have a code of ethics, but groups affiliated with it reportedly summarily executed 21 government soldiers in Aleppo in September, the Times said.
However, Pinheiro said, "It is apparent that the crimes and abuses committed by anti-government groups, though serious, did not reach the gravity, frequency and scale of those committed by the government forces and [the civilian militiamen]."
Human Rights Watch said in its report armed opposition groups subjected detainees to ill-treatment and torture, and committed extrajudicial or summary executions in Aleppo, Latakia and Idlib, which constitute war crimes.
While opposition leaders told the human rights organization they will respect human rights and have taken measures to curb abuses, Human Rights Watch said in a release it had serious concern about statements by some opposition leaders indicating they tolerate such executions.
"Declarations by opposition groups that they want to respect human rights are important, but the real test is how opposition forces behave," said Nadim Houry, deputy Middle East director at Human Rights Watch. "Those assisting the Syrian opposition have a particular responsibility to condemn abuses."
Human Rights Watch recommended that military and civilian Syrian opposition leaders should take all possible measures to end torture and executions by opposition groups immediately, including condemning and prohibiting such practices. Leaders should investigate the abuses, hold those responsible to account, and invite international detention monitors to visit all detention facilities under their control, HRW said.
Initiatives to have armed opposition groups adopt and enforce codes of conduct promoting respect for human rights and international humanitarian law should be encouraged, the organization said.
In addition, countries supplying money or arms to opposition groups should signal they expect strict compliance with international human rights and humanitarian law, Human Rights Watch said.