UN war crimes investigators on Monday asked European countries to "remove barriers" to their work by giving them unhindered access to newly arrived Syrian refugees.
The UN Commission of Inquiry on Syria, which has been investigating human rights violations and war crimes in the country for the past five years, warned that the shift in refugee flows towards Europe had complicated its job.
"It has become more difficult to access victims and witnesses with fresh information," commission chief Paulo Sergio Pinheiro told the UN Human Rights Council in Geneva.
"We are appealing to countries inside Europe hosting newly arrived Syrian refugees to grant us access and remove any barriers to our work," he said, as he presented the commission's latest report to the council.
Pinheiro refused to specify which European nations he was referring to, telling reporters only that the commission had been "in contact with several countries," adding that the investigators had received "assurances that we will have some positive answers."
The investigators have never gained access to Syria itself, instead relying on nearly 4,600 interviews in the region and from Geneva, as well as pictures, medical records and other documents for its findings.
Pinheiro stressed Monday the importance of accessing new arrivals from Syria, who could help the commission document new phases in the drawn-out conflict that has cost more than 300,000 lives.
Their testimonies could prove essential to investigators compiling confidential lists of suspected war criminals and perpetrators of crimes against humanity.
The aim would be to use the information they gather in future prosecution, including at the International Criminal Court.
The commission's appeals to refer Syria to the ICC have however stalled in the UN Security Council.
Commission member and veteran former war crimes prosecutor Carla Del Ponte voiced frustration Monday at the lack of accountability for the horrendous crimes committed in Syria.
"I think it's time that the Security Council is doing something, because it is incredible after five years (there is) no justice for the victims," she told reporters.
Investigator Vitit Muntarbhorn meanwhile said the commission was probing allegations that "incendiary weapons", including phosphorus and napalm, were used in three different locations in Syria.
In its 12th report, published earlier this month, the commission said it was investigating claims that chlorine gas, a chemical weapon, was used in Aleppo in April.
Muntarbhorn said they were also investigating another possible use of chlorine gas last month, but did not specify the location.
A separate UN investigative panel concluded last month that Syrian President Bashar al-Assad's forces had carried out at least two chemical attacks, one in 2014 and one in 2015.
It also found that Islamic State group jihadists had used mustard gas to attack the town of Marea in Aleppo province in August 2015.