UNITED NATIONS - The United Nations human rights chief Thursday warned armed opposition groups in Syria that executions and unlawful killings violate international law and may amount to war crimes, as reports continue to emerge of mass executions in the northern part of the strife-torn nation.

The Office of the High Commissioner for Human Rights (OHCHR) said in a statement that it has received reports in the past two weeks of a succession of mass executions of civilians and fighters who were no longer participating in hostilities in Aleppo, Idlib and Raqqa by hardline armed opposition groups in Syria, in particular by the Islamic State of Iraq and Syria (ISIS).

“While exact numbers are difficult to verify, reliable eyewitness testimony that we have gathered suggests that many civilians and fighters in the custody of extremist armed opposition groups have been executed since the beginning of this year,” High Commissioner Navi Pillay said. Reports suggest that in the first week of January, numerous individuals were executed in Idlib by armed opposition groups. On 6 January, in Aleppo, three individuals who had reportedly been held by ISIS at its base in Makhfar al-Saleheen were found dead, handcuffed, with bullet wounds in their heads.  On 8 January, in Aleppo, numerous bodies, again mostly handcuffed and blindfolded, were found in a Children’s Hospital which had been used as a base by ISIS until it was forced to withdraw after a raid by other armed opposition groups. An eyewitness interviewed by OHCHR identified at least four local media activists among the dead, as well as captured fighters affiliated with various armed opposition groups.

“Information from Raqqa has been more difficult to verify, but there are deeply disturbing reports emerging of mass executions by ISIS when the group withdrew from Raqqa at the beginning of this month, and when it regained control earlier this week,” Ms. Pillay said.

The High Commissioner reminded all parties to the conflict that international law prohibits violence to life and person, in particular murder of all kinds, mutilation, cruel treatment, and torture at any time and under all circumstances.

Meanwhile, The UN-Arab League envoy for Syria Thursday regretted the decision of one of the opposition groups, the National Coordination Committee (NCC), not to take part in next week’s international conference in Switzerland seeking to end nearly three years of civil war.

The envoy, Lakhdar Brahimi, called NCC President Hassan Abdel-Azim on Wednesday to convey his “deep regrets’ but said he respected the decision.

The conference will seek to achieve a comprehensive agreement between the Syrian Government and the opposition for fully implementing the decision of an earlier Geneva conference in June 2012, which called for the creation of a transitional government to hold elections.

Well over 100,000 people have been killed and more than 8 million others driven from their homes since the conflict erupted in March 2011 between the Government and various groups seeking the ouster of President Bashar al-Assad.

The UN has sent out invitations to the conference to more than 30  States, including the five permanent members of the UN Security Council, as well as to representatives of the Arab League, the European Union and the Organization of Islamic Cooperation.

In a statement issued by a UN spokesperson today, Brahimi said he told Dr. Abdel-Azim “he respects the decision of the NCC but deeply regrets that NCC will not be part of the delegation of the Opposition which will discuss in Geneva how the terrible war now raging in Syria must be brought to an end.”

He added that he is familiar with the work of the NCC and its members in support of democracy, and the freedom and dignity of the Syrian people, mentioning in particular two if its members who have disappeared.

There has been no information about Abdel Aziz al-Khair since he was arrested or kidnapped in September 2012 on the road from the airport to Damascus, while Rajaa Al-Nasser was arrested or disappeared in broad daylight in the centre of Damascus on 20 November 2013.

“Mr Brahimi wishes the NCC well, and is confident they will continue to work for the return of peace to their country, and the advent of the new and democratic Syria,” the statement concluded.