The Asayish security forces said 438 kilogrammes (966 pounds) had been seized, equivalent to 2,570,580 pills.

"This is the biggest captagon bust in northeastern Syria," a security official told AFP.

The pills were smuggled into Kurdish-held territory from areas to the west controlled by Turkish-backed Syrian rebels.

"They were professionally concealed in construction materials such as granite, basalt and ceramics," Asayish officials said during a press conference.

A security official told AFP on condition of anonymity that the shipment was seized from a warehouse in Qamishli, the autonomous Kurdish administration's de-facto capital.

The shipment was to be transported to neighbouring Iraq, the official said. Its final destination was not clear.

The bust was the latest in a series of operations targeting captagon shipments from neighbouring areas held by rebel or government forces, the official said.

Captagon was one of the brand names for the amphetamine-type stimulant fenethylline and is now manufactured illegally, mostly in Lebanon and Syria.

The pill is consumed by a wide variety of users, mostly in Gulf countries.

According to a European Union-funded report by the Center for Operational Analysis and Research, "captagon exports from Syria reached a market value of at least $3.46 billion" in 2020.

Seizures have continued to rise and, according to an AFP count, close to 50 million pills have been seized across the region since the start of the year.