Syria's main opposition group has urged the United Nations to probe "numerous massacres" which they say were committed by President Bashar al-Assad's forces during the Muslim fasting month of Ramadan.
The National Coalition also said on Sunday that it was ready to cooperate in any independent investigation into alleged crimes committed by opposition forces.
The Coalition's call comes two days after UN rights chief Navi Pillay said a reported mass execution of Syrian soldiers taken captive by rebels in the northern town of Khan al-Assal should be investigated.
"Assad has violated international humanitarian and human rights laws. He has committed crimes against humanity by using ballistic missiles, toxic gas, fixed-wing aircraft and chemical weapons," the opposition said in a statement.
"Assad's forces have summarily executed civilians, burned them, and targeted towns, villages, schools, hospitals, shelters, and places of worship across Syria," it added.
Pillay said on Friday that the events during fighting for control of the town of Khan al-Assal in Aleppo province were "deeply shocking".
Video footage shot by rebels and reports provided by the Syrian Observatory for Human Rights suggest dozens of members of regime forces were executed.
"There needs to be a thorough independent investigation to establish whether war crimes have been committed. And those responsible for such crimes should be brought to justice," Pillay said.
"Opposition forces should not think they are immune from prosecution. They must adhere to their responsibilities under international law," she added.
The opposition said it was ready to cooperate in any independent probe. The Coalition "reiterates its full commitment to international conventions, emphasising that those who violate international human rights law must be brought to justice regardless of who they might be", the statement said.
The group "further expresses its readiness to cooperate with all independent organisations to conduct impartial investigations into the war crimes committed across Syria."