"It's going to new lows that these banned weapons are being used and civilians and children are being killed," said Kimberly Brown, a conflict adviser with Save the Children.
The organization is working with refugees in countries around Syria, including Iraq, Jordan and Lebanon.
How did the Syrian crisis begin?
What started as security forces cracking down on mostly nonviolent protesters has spiraled into a civil war between pro-government forces and the rebels, including the Free Syrian Army.
About 40,000 civilians have been killed since the first protests 20 months ago against President Bashar al-Assad's government, according to the opposition Center for Documentation of Violations in Syria. And more than 380,000 Syrian refugees have fled to neighboring countries, creating humanitarian challenges abroad.
The Syrian government routinely refers to its battle against "terrorists," the term it uses for rebel fighters and extremist elements in the country.
CNN cannot confirm claims by the government or the opposition because of government restrictions that prevent journalists from reporting freely within Syria.