The armed opposition and government in Syria both stand accused of committing severe human rights abuses in the 18-month uprising against President Bashar al-Assad's government, the United Nations (UN) and Human Rights Watch (HRW) say.
HRW, a US-based group, has accused the opposition of having subjected detainees to torture and committed extrajudicial or summary executions in several areas.
In a report released on Monday, the group said that such incidents occurred in the northern provinces of Idlib and Aleppo, as well as the coastal region of Latakia.
It said that such acts in the context of an armed conflict were war crimes, and could constitute crimes against humanity if they occurred in a widespread and systematic manner.
The group documented more than a dozen extrajudicial and summary executions by opposition forces.
HRW said three opposition leaders who were confronted with evidence of extrajudicial executions said those who killed deserved to be killed, and justified the acts by saying that executions were reserved for the "worst criminals".
UN war crimes list
Meanwhile, the UN's human rights investigators said on Monday that they had drawn up a new secret list of individuals and army units suspected of having committed war crimes in Syria and who should face criminal prosecution.
The independent investigators, led by Paulo Pinheiro, said they had gathered "a formidable and extraordinary body of evidence" and urged the UN Security Council to refer the situation in Syria to the International Criminal Court (ICC).
"A second confidential list of individuals and units believed to be responsible for violations is being provided to the High Commissioner of Human Rights," Pinheiro told the UN Human Rights Council in Geneva.
The investigators accused government forces, both formal and informal, as well as the anti-government armed opposition of committing war crimes and crimes against humanity, including murder, summary executions, torture.
Moreover, the UN confirmed that "foreign elements" were increasingly playing a role in the conflict in Syria.
Pinheiro said there was an "increasing and alarming presence" of religious extremists in Syria, some joining the rebels and others operating independently.
Their presence tended to radicalise the rebels who have also committed crimes, he said.
Western countries are seeking yet another condemnation of President Assad's government at the session.
"The international community must ensure impunity will not prevail," Mariangela Zappia, the European Union's ambassador, said at Monday's debate that was also attended by Syrian envoy Faysal Khabbaz Hamoui.
The commission said that it would not be publically releaseing the names on its list, due to the lower standard of proof employed by commissions of inquiry as compared to courts of law, Pinheiro said.
Meanwhile, in Syria, fighting continued in Aleppo on Monday, amid a disputed claim that the government had seized the strategic Midan district from rebels, an anti-government rights group said.
The army clashed with rebel fighters in several districts of the northern city, which has been the site of fierce fighting since July 20, the Syrian Observatory for Human Rights (SOHR) said.
One battle broke out near a building housing the feared air force intelligence service, and rebels also attacked a military post in the New Aleppo area, the UK-based group said.
Al-Watan, a pro-government newspaper, reported that the army had "cleansed" the Midan district, where the "majority of the armed men were Arab and foreign extremists".
But Rami Abdel Rahman, the SOHR's director, said that account was untrue.
"Clashes are ongoing in Midan and in several parts of Aleppo. Such claims are just part of the media war," he said.
Shelling was also reported in the al-Hajar al-Aswad district of Damascus.
Brahimi's Cairo meetings
Also on Monday, Lakhdar Brahimi, the joint UN-Arab League envoy to Syria, was due to hold talks with Nabil al-Arabi, the Arab League chief, in Cairo.
"Brahimi will brief Arabi on his visit to Syria and his meeting with Syrian President Bashar al-Assad and members of the opposition there," an Arab League official said.
The UN-Arab League envoy and Arabi will discuss "the Arab and international moves required to resolve the current crisis in Syria," the official said.
The veteran Algerian and international diplomat left Damascus on Sunday, warning that worsening conflict in Syria threatens both the region and the world.
"The crisis is dangerous and getting worse, and it is a threat to the Syrian people, the region and the world," said Brahimi, who replaced Kofi Annan following the failure of the former UN chief's six-point peace plan.
As Brahimi left Damascus, a rebel commander who had an Internet conference call with the envoy on Sunday said his mission was doomed to fail.
"We are sure Brahimi will fail like the other envoys before him, but we do not want to be the reason of his failure," Colonel Abdel Jabbar al-Okaidi, the Free Syrian Army chief for Aleppo province, told the AFP news agency.
According to the Iranian Fars news agency, Brahimi is also expected to attend part of a meeting of foreign ministers of a Syria "contact group" in Cairo. The group includes Egypt, Saudi Arabia, Iran and Turkey.