Six people have been charged with human trafficking after 18 Afghan migrants were found dead inside a truck dumped on a dirt road in Bulgaria.

Officials said the driver had earlier refused to stop the truck despite strong and prolonged banging from the 52 people in hidden compartments.

The head of the National Investigative Service Borislav Sarafov said the 18 victims died of a combination of lack of oxygen in an enclosed space and difficulty breathing as they had been crammed into the truck “like in a tin can”.

“The victims died slowly and painful.”

“This case shows an extreme callousness and demonstrates that migrants are seen only as goods that should be shipped from one place to another, irrespective of whether they are alive or dead,” he said.

The other 34 migrants – found on Friday near the capital Sofia – remain in a stable condition in hospital.

Five of those charged are in custody, while one of the suspected traffickers, who had managed to flee the country, is being sought with a European arrest warrant.

Prosecutors said the ring had trafficked migrants from the border with Turkey across Bulgaria to Serbia, from where they continued their journey mainly to Britain, Germany, and France.

The truck driver and his companion were also charged over the deaths.

The incident has shocked Bulgaria, in what is one of the worst incidents of its kind on the overland route across the Balkans into Europe.

Bulgaria is a gateway for migrants coming into the European Union through Turkey. The government has been accused of mistreating refugees as they attempt to enter the continent.

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Sky News reported last year that officials near the Bulgaria-Turkey border fence had shot an asylum seeker in October after they caught a group of people illegally entering the country.

Bulgaria is part of the European Union and hopes to get membership in the Schengen area, which allows people to move freely across borders within it.

Last year, more than 150,000 people attempted to cross the border with Turkey, which was more than four times higher than the previous year.