Investigations have been launched following allegations that Syrian government forces used chemical weapons in attacks around a city held by rebel fighters.
Human Rights Watch reported that evidence emerging from Syria pointed to toxic chemicals being used in barrel bombs during attacks against the northern city of Idlib between 16th and 31st March.
HCR said that Syrian rescue workers reported the attacks killed at least six people, including three children, and affected more than 200 others.
The organisation has launched inquiries into six attacks and claimed that witness accounts, backed up by photographic and video evidence, "strongly indicated" a chemical attack in three of the bombings.
"While Human Rights Watch cannot conclusively establish the chemical used, several witnesses described a chlorine smell," the HCR website said, adding: "Syrian government forces have previously dropped barrel bombs embedded with cylinders of chlorine gas."
An account of one attack from a rescue worker in the area detailed how six members of the same family died when bombs were dropped on Sarmin, a small town near Idlib.
Leith Fares, a rescue worker with Syrian Civil Defence, told HCR that his team had heard a family had become trapped in their basement after the attack. When they reached the house, Fares said he found it difficult to breathe in the basement and thought the family had died.
"They looked dead, but they had no visible wounds," he said.
The family were taken to hospital where all of them died, according to Dr Mohamed Ghaleb Tirani, the director.
"The children were foaming at the mouth, they were suffocating, then their hearts stopped. The parents had trouble breathing. We tried to treat them, but they died as well," he said.
The allegations from HCR come days after the Oganisation for the Prohibition of Chemical Weapons announced it was analysing the same allegations of the attacks on Idlib.
"We have been monitoring the recent reports suggesting that toxic chemicals may have been used as weapons in the Idlib province of the Syrian Arab Republic," OPCW Director-General, Ambassador Ahmet Üzümcü, said.
"This matter is of serious concern."
The chemical weapons watchdog has a United Nations Security Council mandate to look into all serious allegations of use of toxic chemicals for hostile purposes in Syria.