New footage obtained by the Middle East Media Research Institute purports to show a weapon carrying the chemical agents fired at a Damascus suburb on the morning of August 21, killing more than 1,400 people, according to US figures.

The video, according to MEMRI, is claimed by “pro-opposition internet sources” as showing the attack on Damascus’ Ghoutta district that day.

The clip of more than five minutes, which is unverified, shows about a dozen men in what appear to be green military fatigues positioning a type of large missile mounted on a truck, by the side of a road.

After several long minutes of aiming, the men slowly leave the area and the missile is somehow fired. What sound like screams and cries can be heard in the distance, as the camera frame fills with heavy smoke.

Once the smoke clears, the footage shows men gradually walking back to the site.

The video was published on August 28, amid increased talk of international intervention in Syria, and a day before the UK parliament rejected any such option of participating in a US-led attack, handing Prime Minister David Cameron a crushing defeat.

The US claims it has new evidence confirming sarin gas was used in the attack. “We have learned through samples that were provided to the United States, that have now been tested, from first responders in east Damascus, and hair samples and blood samples have tested positive for signatures of sarin,” US Secretary of State John Kerry said Sunday, a day after President Barack Obama stepped back from his threat to attack Syria in response to the alleged chemical attack, saying he would first seek authorization from Congress.

The secretary said that Syria’s President Bashar Assad “has now joined the list of Adolf Hitler and Saddam Hussein” in deploying chemical weapons against his population and that “the case remains the same” for a US response. Kerry echoed Obama in saying the world cannot stand by and watch Assad use chemical weapons.