A suspected chemical attack on a rebel-held town in Syria's Idlib province has drawn widespread international condemnation, with the United Nations saying it will investigate the bombing raid as a possible war crime.
SNHR documented that at least 500 people, including 250 children, were killed in three days.
SAMS doctors on the ground said the attack caused people to vomit and foam from the mouth. Others lost consciousness and suffered muscle spasms.
The group said the symptoms, which also included constricted pupils and slow heart rate, were indicative of an organo-phosphorus compounds agent - a category of toxic gases which includes sarin.
'Toxic gas attack' kills at least 58 people in Syria
"We were affected by the gas. We couldn't stand up," Veda Ajej, one of the survivors treated in a hospital in the Turkish town of Reyhanli, near the Syria border, told the Reuters news agency.
"I felt dizzy and nauseous," she said, lying in a hospital bed. "I couldn't breathe."
Local health workers said the death toll could rise and eventually reach 100. A member of the White Helmets, a rescue group that operates in rebel-held areas, told Al Jazeera that up to 300 people had been injured.
The Syrian National Coalition, an opposition group, said a gas similar to sarin was used in the attack, which it blamed on government planes.
Syria's military rejected the accusation, saying in a statement that the army "denies using any toxic or chemical agents in Khan Sheikhoun today, and it did not and never will use it anywhere".
Russia, an ally of Syria's President Bashar al-Assad, on Wednesday said Syrian aircraft had carried out a raid, but the chemicals were part of a "terrorist" stockpile of "toxic substances" that had been hit on the ground.
Save the children of Syria
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