(CNN) United Nations human rights experts are investigating reports of alleged chlorine bomb attacks on civilians in two Syrian towns, officials said Tuesday.

"Most alarmingly, the commission has received multiple reports, now under investigation, that bombs allegedly containing weaponized chlorine have been used in the town of Saraqeb in Idlib and in Douma in Eastern Ghouta," near Damascus, Paulo Pinheiro, chair of the UN's commission of inquiry on Syria, said in a statement issued in Geneva.

The panel also expressed "deep concern" over the escalation of violence in rebel-held Idlib province and in Eastern Ghouta, Pinheiro said.

The White Helmets, a volunteer rescue group, said three of its members and six others were injured by a chlorine gas attack in Idlib's Saraqeb on Sunday night. The group posted several videos on social media showing men coughing and being put onto stretchers.

Two media activists who spoke to CNN from the nearby town of Kafranbil said they were told that the chlorine attack in Saraqeb had been launched from Syrian helicopters.

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