Syrian rebels
Syrian rebelsReuters

Syrian rebels, including Al-Qaeda's former affiliate in the country, vowed on Tuesday to avenge the deaths of dozens of people in a suspected gas attack, calling on allied fighters to "ignite the fronts", AFP reported.

"We call on all the fighters of Sham (Syria) to ignite the fronts," the Tahrir al-Sham alliance said in a statement posted online and quoted by the news agency.

"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 added.

The chemical attack took place Tuesday morning in 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.

A source in Washington said later on Tuesday that the U.S. government believes the chemical agent sarin was used in the chemical attack in Idlib, and that it was "almost certainly" carried out by forces loyal to Syrian President Bashar Al-Assad.

The Syrian military denied responsibility and said it would never use chemical weapons.

The Organization for the Prohibition of Chemical Weapons (OPCW) has in the past found that some people in Syria may have been exposed to sarin or a sarin-like gas.