In their latest report, the team led by Paulo Pinheiro said that it was unable to determine who carried out a massacre of more than 100 people in Houla in May, but "forces loyal to the government may have been responsible for many of the deaths".
It also had multiple reports of killings by armed opposition groups who are increasingly using improvised explosive devices in their revolt against the rule of President Bashar al-Assad.
"The situation on the ground is dangerously and quickly deteriorating," the 20-page report said, citing government use of machine guns, artillery and tanks in shelling restive areas including the city of Homs.
"In the increasingly militarised context, human rights violations are occuring across the country on an alarming scale during military operations against locations believed to be hosting defectors and/or those perceived as affiliated with anti-government armed groups, including the 'Free Syrian Army'."
The independent U.N. investigators voiced strong concerns that rebels were "using children as medical porters, messengers and cooks, exposing them to risk of death and injury".
Some had been going back and forth across the border with Turkey.
Pinheiro, who made a first visit to Damascus at the weekend for talks with senior Syrian officials, presented the report to the U.N. Human Rights Council whose debate was to follow.
He discussed the Houla investigation with Syrian authorities and believed that a U.N. commission would be able to begin working inside Syria, he told the 47-member Geneva forum.
The team, which conducted nearly 400 interviews, said it had collected photographs, videos, satellite imagery and documentary evidence during its recent investigative missions in the region.
It was updating its secret list of identified perpetrators for possible use by a future mechanism of criminal justice.
"As a result of the reported flow of new weapons and ammunitions both to the government forces and to the anti-government amred groups, the situation risks becoming more aggravated in the coming months," Pinheiro said.