UN investigators say they have "reasonable grounds" to believe chemical weapons have been used by both sides in Syria, and warn that crimes against humanity are now occurring daily in the war-torn country.

"Allegations have been received concerning the use of chemical weapons by both parties," said the Commission of Inquiry on Syria in a report to the UN Human Rights Council on Tuesday, adding that "the majority concern their use by government forces".

There are "reasonable grounds to believe that chemical agents have been used as weapons," added the investigators.

It was the first time the commission, which has been tasked with probing human rights violations in Syria since 2011, added the suspected use of chemical agents to its long list of war crimes committed in the country.

Without providing details, the report lists four chemical attacks: on the Khan al-Asal neighbourhood of Aleppo and Uteibah, near Damascus, both on March 19, on the Sheikh Maqsood district of Aleppo on April 13, and on the northwestern town of Saraqab on April 29.

"It has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator," said the report, which covers the period January 15 to May 15 this year.

Carla del Ponte, a legendary former war crimes prosecutor and a member of the commission, warned however that focusing too heavily on the issue of chemical weapons could overshadow the overall suffering in a country where more than 94,000 people have been killed in more than two years of violence.

"We have so many deaths in Syria now, ... so please don't make the use of chemical weapons in Syria now the most important issue," she told reporters ahead of Tuesday's presentation.

The report pointed out that "war crimes and crimes against humanity have become a daily reality in Syria".

During the four-month period covered by the report, 17 suspected massacres took place in Syria, out of a total of 30 since the conflict began in March 2011, the investigators said.

Damascus has so far barred the commission, headed by Brazilian Paulo Sergio Pinheiro, from the country, and the investigators have instead based their conclusions on more than 1600 interviews with refugees and exiles, as well as by Skype and telephone with victims and witnesses inside the country.

"I am very surprised by the violence and the cruelty, the criminal acts and especially the acts of torture" in Syria, said del Ponte, also decrying the use of child soldiers by some opposition groups.

According to the report, 86 child combatants have so far been killed in the conflict - nearly half of them in 2013.

Another development, according to Pinheiro, was the systematic imposition of sieges and forced displacement as weapons of war.

The commission, which has drawn up a confidential list of suspected perpetrators, has repeatedly called on a deadlocked UN Security Council to refer the situation in Syria to the International Criminal Court in the Hague.

The investigators also called on the international community to refrain from shipping arms to Syria.

"It is an illusion that more weapons will tip the balance between the two parties," Pinheiro told reporters, insisting that "more weapons will only lead to more civilian deaths and wounded".

Del Ponte cautioned that international arms suppliers "could carry joint criminal responsibility" for violations committed with the weapons.

Tuesday's report accuses the Syrian army and its allies, including foreign fighters like Lebanese Hezbollah, of suspected "atrocities against women and children," murders, torture, and forced displacements, among other crimes.

The report also accuses armed opposition groups of a long line of war crimes, including summary executions and torture, but reiterates that their violations had not reached the intensity and scale of those committed by the regime.