War crimes by both government forces and rebels in Syria are spiralling amid an increasingly radicalised and sectarian conflict, a UN-appointed inquiry team said on Monday.

"The depth of the Syrian tragedy is poignantly reflected in the accounts of its victims. Their harrowing experiences of survival detail grave human rights violations, war crimes and crimes against humanity," the Independent International Commission of Inquiry said in a 131-page report.

"The destructive dynamics of the civil war not only have an impact on the civilian population but are also tearing apart the country's complex social fabric, jeopardising future generations and undermining peace and security in the entire region," it warned.

Set up in 2011 by the UN Human Rights Council, the commission includes the world body's former war crimes prosecutor, Carla del Ponte.

It has been unable to gain access to Syria, where President Bashar al-Assad's regime is battling to put down a revolt that erupted in March 2011 and which the United Nations says has now claimed 70,000 lives.

In a report last August -- based on more than 1,000 interviews with perpetrators and victims -- it blamed both sides for war crimes, albeit the rebels to a lesser extent.

The latest report, for which the commission interviewed some 450 people, said the pattern had worsened amid the spiralling violence.

"The situation of human rights in the Syrian Arab Republic has continued to deteriorate," it said.

"The conflict has become increasingly sectarian, with the conduct of the parties becoming significantly more radicalised and militarised."

Most of the rebels fighting the regime are Sunni Muslims, while the ruling clan and many of its most fervent supporters are from the minority Alawite sect, an offshoot of Shiite Islam.

"Government forces and affiliated militia committed the crimes against humanity of murder, torture, rape, enforced disappearance and other inhumane acts," the report said.

The commission likewise lambasted the rebels.

"Anti-government armed groups have committed war crimes, including murder, torture, hostage-taking and attacking protected objects. They continue to endanger the civilian population by positioning military objectives inside civilian areas," it said.

"Where armed groups carried out bombings in predominantly civilian areas, it had the effect of spreading terror and amounted to the war crime of attacking civilians," it added.

Both sides have violated international standards on the rights of children by using them in the conflict, it said.

Despite criticising both camps, the report underlined that the regime remained the major perpetrator.

"The violations and abuses committed by anti-Government armed groups did not, however, reach the intensity and scale of those committed by government forces and affiliated militia," it said.

The commission's current mandate runs out next month, when it is to give UN human rights Navi Pillay a confidential list of names of individuals and units to blame, setting the wheels of international justice in motion.

"Ensuring the accountability of all parties for crimes committed is imperative," the report said, urging the international community to remain vigilant.

"The quest for peace and justice is a shared responsibility at the national, regional and international levels," it added.

"There is no military solution to the conflict. A sustained cessation of hostilities remains of paramount importance to end the violence and gross human rights violations and abuses."