Rebels in southern Syria seized – then destroyed - $1.4m worth of drugs while conducting an operation against the Islamic State (IS) group, the US-led coalition backing them said on Monday.

According to the coalition, the narcotics were captured on 31 May by Maghawir al-Thawra, a rebel group trained by the United States and Jordan, during an offensive against IS in southeast Syria.

The drugs’ street value was estimated to be worth around $1.4m.

'The cache included more than 300,000 pills of Captagon, an illegal drug frequently trafficked and used by Daesh members' - US-led coaliton

"The cache included more than 300,000 pills of Captagon, an illegal drug frequently trafficked and used by Daesh members," the coalition said, using the Arabic acronym for IS.

Maghawir al-Thawra (Commandos of the Revolution) has been conducting operations against IS-affiliated groups in the Syrian desert near Al-Tanf, a base on the Jordanian border used by US and British troops.

Captagon, an amphetamine usually found in pill form, has grown in prominence over the Syrian war.

It has been used extensively by fighters throughout the seven-year war. The drug numbs the senses and can keep its users awake for days, which fighters say is useful in the battlefield.

Predominantly manufactured in Syria, neighbouring countries such as Lebanon have also begun to see a rise in Captagon being produced within their borders. The amphetamine is also used in Syria and elsewhere as a party drug.

In 2015, a Saudi prince, Abdel Mohsen Bin Walid Bin Abdulaziz, was caught, along with four others, attempting to smuggle two tonnes of Captagon out of Beirut airport.

The pills were packed up and waiting to be loaded onto a private jet for transportation to Saudi Arabia when the Lebanese authorities stepped in.

'End with certain death'

On Friday, Maghawir al-Thawra released a slick video on its Facebook page announcing the capture and destruction of the stash.

It showed images of many parcels said to contain drugs, as well as thousands of pills being doused with fuel and set alight.

The coalition said the drugs were seized within a 55km deconflicton zone around al-Tanf used as a buffer between US-backed forces and those loyal to the Syrian government.

“We will work on cleansing the 55km area from all types of drugs,” text in the video reads. “And we will deal with these dealers and smugglers, legally and justly.”

'Drugs are the enemy of health, family and society' - Maghawir al-Thawra

“Drugs are the enemy of health, family and society,” it adds. “They begin with addiction and end with certain death.”

US and British forces have used al-Tanf as a base from which to train allied Syrian rebels and advise them.

“This is how the Army of Commandos of the Revolution combats and eliminates the harmful and corruptive substances that plague and murder our family and society,” a statement from the rebel group’s leadership said.