Thai wildlife authorities said they have rescued over 100 long-tailed macaques that were being illegally smuggled in the back of a pick-up truck, but more than a dozen did not survive.

The truck was intercepted on Thursday at a checkpoint while on its way to a Thai province bordering Cambodia, transporting 102 macaques in blue mesh bags which were stashed together in various plastic baskets.

The monkeys could be seen struggling and gasping for air when authorities found them, according to video footage provided to Reuters by the Department of National Parks, Wildlife and Plant Conservation (DNP).

Thai veterinarians set up a field hospital, equipped with ultrasound scanners and breathing tubes, to treat 80 of the macaques. Four were in critical condition while 18 did not survive, authorities said.

Some of the monkeys were pregnant.

A field hospital was set up to treat the monkeys

"The monkeys were exhausted and dehydrated for a long time. Some of them had respiratory problems because of overcrowding during the smuggling operation," the department's head vet Pattarapol Maneeon said.

The driver of the truck was being detained for wildlife smuggling.

The man told local media that he was hired for 3,000 baht (€79) to drive the truck to the border and that he was not aware there were monkeys in the back of the vehicle.