Syrian opposition groups vowed on Tuesday to avenge the deaths of dozens of people who were killed in a suspected chemical gas attack.
At least 58 people, including 19 children, were killed early on Tuesday morning after air strikes on a town in northwest Idlib Province, most of which is held by opposition Tahrir al-Sham alliance.
"We call on all the fighters of Sham (Syria) to ignite the fronts," the group said in a statement posted online.
The UN Security Council will meet on Wednesday to discuss the attack. Britain, France and the United States on Tuesday presented a draft resolution to the council condemning the attack and demanding a full investigation as soon as possible.
The UN Commission of Inquiry for Syria said it had begun investigating the attack, adding that reports of the use of chemical weapons was “extremely concerning."
The Organisation for the Prohibition of Chemical Weapons (OPCW), based in The Hague, also said it was "gathering and analysing information from all available sources" about the attack.
The Assad regime denied responsibility for the attack and said it would never use chemical weapons.
Speaking to Reuters, one Syrian opposition member said the attack was a "direct consequence" of recent US statements saying that it was no longer focused on making Bashar al-Assad leave power.
"The first reaction from Syrians is that this is a direct consequence of American statements about Assad not being a priority and giving him time and allowing him to stay in power," Basma Kodmani said.
Meanwhile, the United States warned on Tuesday that Bashar al-Assad must be held accountable for chemical weapons attacks and demanded that Russia and Iran bring their ally to heel.
"While we continue to monitor the terrible situation, it is clear that this is how Bashar al-Assad operates: with brutal, unabashed barbarism," US Secretary of State Rex Tillerson said.
Russia and Iran have backed the Assad regime throughout the Syrian civil war, which began in March 2011. Russian fighter jets often carry out airstrikes on opposition-held areas on behalf of the regime.
Russia denies involvement
NATO member Turkey, a key mediator alongside Russia and Iran in peace talks between Syrian regime and opposition representatives in the Kazakh city of Astana, condemned the attack.
Turkish presidential sources said that Russia had categorically denied any involvement in the incident when Turkish President Recep Tayyip Erdogan touched on the issue in a conversation with his Russian counterpart Vladimir Putin.
The two leaders also emphasised the importance of maintaining an ongoing ceasefire in Syria.