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At least 58 people, including 11 children, were killed in an air raid that released "toxic gas" on the rebel-held Syrian town of Khan Sheikhun on Tuesday, a monitor said.
Tuesday's attack caused many people to choke or faint, and some had foam coming out of their mouths, the Britain-based Syrian Observatory for Human Rights said, citing medical sources who described it as a sign of a gas attack.
Damascus has repeatedly denied using such weapons.
The monitoring group, which monitors the war through a network of contacts on the ground, was unable to confirm the nature of the substance.
Syrian government or Russian jets pounded the town of Khan Sheikhoun in rebel-held Idib in the morning, the monitoring group said.
The Syrian army could not immediately be reached for comment..