Crimes against humanity are happening every day in Syria, UN investigators say, with mounting evidence that both sides have committed massacres, engaged in torture and used chemical weapons.

The UN Commission of Inquiry on Syria said on Tuesday that military and government leaders must be held accountable for implementing a "concerted policy" of human rights violations.

"War crimes and crimes against humanity have become a daily reality in Syria where the harrowing accounts of victims have seared themselves on our conscience," its report said.

"There is a human cost to the increased availability of weapons," it added.

Investigators said they had "reasonable grounds" to believe that limited amounts of chemical weapons had been used in Syria.

In their latest report based on interviews with victims, medical staff and other witnesses, they said they had received allegations that Syrian government forces and rebels had used the banned weapons, but that most testimony related to their use by President Bashar al-Assad forces.

The inquiry said conclusive findings could be reached only after testing samples taken directly from victims or the site of the alleged attacks. It called on Damascus to allow a team of experts into the country.

In an apparent message to European countries considering arming Syrian rebels, the report said the transfer of arms would heighten the risk of violations, leading to more civilian deaths and injuries.

UN Secretary-General Ban Ki-moon had earlier appointed a UN team to investigate alleged chemical weapons attacks after the Syrian government asked him to investigate a purported attack by rebels on March 19 on Khan al-Assal village in the northern province of Aleppo.

While the Syrian government insisted that an investigation be limited to that incident, Ban pressed for a broader investigation, including a December incident in Homs. Investigators were eventually not allowed into the country.

Any confirmation of the use of chemical weapons may escalate the international response to the more than two-year-old conflict, which has killed more than 94,000 people, according to UN estimates.

The latest UN report, covering the period from mid-January to mid-May, accused both sides of committing war crimes.

The report accused government forces and affiliated militia of committing torture, rape, forcible displacement and enforced disappearance.

It also accused rebels of carrying out executions without due process, as well as committing torture, taking hostages and pillaging.

But it said violations and abuses by the rebels "did not, however, reach the intensity and scale of those committed by government forces and affiliated militia".

"A dangerous state of fragmentation and disintegration of authority prevails in areas under anti-government armed groups control, despite attempts to fill the vacuum left by the withdrawal of the state through creating local councils,'' it said.