In a gruesome incident, at least 58 people, including 11 children, lost their lives and over 150 others were injured in a suspected chemical attack on a rebel-held town in Syria, media reports said on April 4. According to the reports, the attack struck the town of Khan Sheikhoun in Idlib province, which is largely controlled by an alliance of rebels including former Al-Qaeda affiliate Fateh al-Sham Front.

It was the third claim of a chemical attack in just over a week in Syria. The previous two were reported in Hama province. This chemical attack left hundreds suffering from respiratory problems and showed the symptoms like vomiting, fainting and foaming at the mouth, medical teams have said.

Soon after the attack, social media site Twitter got flooded with pictures and videos showing people, including children, became unresponsive and others can be seen struggling to breathe or wearing oxygen masks.

Chemical weapons attacks are not uncommon in Syria’s conflict. According to the UN report, the worst attack was an attack by toxic sarin gas in August 2013 on the Damascus suburb of Ghouta that killed more than 1,400 civilians.

Soon after the attack, the then United States President Barack Obama asked the Congress to authorize use of military action against Syria. Shortly after Obama’s statement, President Bashar Assad agreed to Russia-sponsored deal to destroy his chemical arsenal and prompted Obama to retreat his order of military action against the country.

According to the reports, Bashar Assad declared a 1,300-ton stockpile of chemical weapons and other supplements that can be used to make weapons.

After the declaration, those weapons were destroyed, but member states of the Organisation for the Prohibition of Chemical Weapons (OPCW) have repeatedly questioned whether Assad declared everything in 2013.

Meanwhile, a joint investigation by the United Nations and the international chemical weapons watchdog revealed that Syrian government was behind at least three attacks in 2014 and 2015 involving chlorine gas and the Islamic State group was responsible for at least one involving mustard gas.

Since the war began in 2011, an estimated 400,000 Syrians have been killed, according to figures issued by the UN last year.