Syrian army has blamed "terrorist groups" for using "chemical and toxic substances".
Syrian rebels including Al-Qaeda's former affiliate vowed on Tuesday to avenge the deaths of dozens of people in a suspected gas attack, calling on allied fighters to "ignite the fronts".
"We call on all the fighters of Sham (Syria) to ignite the fronts," the Tahrir al-Sham alliance said in a statement posted online.
"We promise the criminal regime and its allies revenge that will soothe the hearts of our people in Khan Sheikhun in particular, and Syria in general," the statement said.
At least 58 people, including 19 children, were killed early Tuesday morning after air strikes on Khan Sheikhun in northwest Idlib Province, most of which is held by Tahrir al-Sham.
The alliance is dominated by the Fateh al-Sham Front, which was known as Al-Nusra Front before it cut its ties to Al-Qaeda.
The Syrian Observatory for Human Rights, which reported the death toll, said another 160 people were wounded in the raids on Khan Sheikhun.
The monitoring group and local medics told AFP that those affected were suffering from difficulty breathing, rapid pulses, pinpoint pupils, and foaming at the mouth.
Syria's political opposition blamed President Bashar al-Assad's forces, saying the attack cast doubt on the future of peace talks.
But the army denied any involvement, blaming "terrorist groups" for using "chemical and toxic substances".
The United Nations Commission of Inquiry for Syria said it had opened up an investigation into the allegations, which if confirmed would be one of the worst chemical attacks since Syria's civil war erupted in 2011.
Damascus officially joined the Chemical Weapons Convention and turned over its declared chemical arsenal in 2013, as part of a deal to avert US military action after chemical weapons strikes allegedly carried out by government troops.
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