A UN human rights team has collected testimonies that Syrian rebels have used the nerve agent sarin, a Swiss investigator said in remarks published online on Monday, dpa reported.
"According to the testimonies we have gathered, the rebels have used chemical weapons, resorting to sarin gas," Carla del Ponte said on Sunday in an interview with Swiss public television.
Del Ponte is a member of the Commission on Inquiry on Syria, which is scheduled to issue a new report on human rights violations in the country for the UN Human Rights Council in Geneva late this month, a council spokesman told dpa.
She added that her commission would probe wether the regime of Syrian President Bashar al-Assad also used the chemical weapon.
"The investigations are far from being completed," del Ponte told Swiss Radio and Television (SRF), which published her remarks online.
"Our investigations will need to be further developed, verified and ascertained, but, according to what we were able to establish, for the moment only the opposition to the regime has used sarin gas," she told SRF's Italian-language branch.
The rebels and the government of President Bashar al-Assad have accused each other of using chemical weapons. The United States, Britain and Israel have also said there was proof that such weapons were used in Syria.
US President Barack Obama said on Wednesday that his administration would not take any action before it had established exactly who has used chemical weapons in Syria.
US officials have identified the substance as sarin - a highly toxic nerve agent that can cause quick paralysis or death.
The UN commission's update report is to be discussed at the Geneva council on June 3 or 4.