Ignite the fronts: Syrian rebels seek gas attack vengeance

In a statement posted online, the Tahrir Al-Sham alliance urged a revenge attack after scores of people, including at least 20 children were killed in airstrikes on Khan Sheikhun in the northwest Idlib province on Tuesday.

"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 read.

“We call on all the fighters of Sham (Syria) to ignite the fronts," they added.

The United Nations Commission of Inquiry for Syria said it had opened up an investigation into the allegations of the use of chemical weapons. If the allegations are confirmed, it would be branded as one of the worst chemical attacks since Syria's civil war erupted after Assad crushed peaceful revolutions calling for regime change in 2011.

Victims of the attack suffered symptoms that are typical to a gas attack, including respiratory problems and symptoms including vomiting, fainting, rapid pulses, pinpoint pupils and foaming at the mouth, according to the Syrian Observatory for Human Rights monitor group.

The Assad regime had denied carrying out the attacks, and had blamed the "terrorist groups" for using "chemical and toxic substances".

Russia also defended its ally Damascus on Wednesday, saying a Syrian airstrike hit a "terrorist warehouse".

"According to the objective data of the Russian airspace control, Syrian aviation struck a large terrorist warehouse near Khan Sheikhun," the Russian defence ministry said in a statement.

It housed "a warehouse making bombs, with toxic substances", said the ministry, without stating if the strike was voluntary or deliberate.

Syria had officially joined the Chemical Weapons Convention and turned over its declared chemical arsenal in 2013 after the deadly Ghouta attack, as part of a deal to avert US military action after chemical weapons strikes allegedly carried out by government troops.

Agencies contributed to this story