United Nations officials have voiced deep concern at reported mass killings of civilians in Syria saying such attacks may constitute crimes against humanity.

The Special Advisers of the Secretary-General on the Prevention of Genocide, Francis Deng and Edward Luck, expressed grave alarm at widespread reports of mass killings in attacks that involved a series of Government artillery and tank shelling on residential neighbourhoods in Syria, as well as alleged attacks against civilians and civilian infrastructure by a pro-government militia and other armed groups.

These "may constitute crimes against humanity. These massacres underscore the Syrian Government's manifest failure to protect its population," they said in a joint statement issued here, late on Thursday.

"The Special Advisers join the many voices that have condemned these attacks and reiterate their calls for all parties to immediately end all acts of violence and human rights violations against the Syrian population, irrespective of their ethnicity, religion or political affiliation and commit to implementation of the Joint Special Envoy's six-point plan," they said.

They called on the international community to "take immediate, decisive action to meet its responsibility to protect populations at risk of further atrocity crimes in Syria, taking into consideration the full range of tools available under the United Nations Charter."

Given the extreme gravity of the crimes committed, the Special Advisers urged the Security Council to consider the request of the High Commissioner for Human Rights to refer the situation in Syria to the International Criminal Court.

"There can be no lasting peace without full accountability," they said.

With the increasing violence and deepening sectarian tensions, they added, the "risk of further mass atrocity crimes is high. The time for action is now." They stressed that the mass killings of civilians in El-Houla and Mazraat al-Qubeir, including the brutal assault and murder of women and children at close range, represent an "alarming escalation in targeted attacks against civilians.

They must cease immediately." Recalling the commitment by all Heads of State and Governments at the 2005 World Summit to protect populations from genocide, war crimes, ethnic cleansing and crimes against humanity, including their incitement, the Special Advisers underlined the importance of unimpeded access for UN observers in Syria to carry out their mandate, particularly in ascertaining the facts of such killings.