Bulgarian prosecutors have charged 6 people with human trafficking after 18 Afghan migrants were found dead inside a truck dumped on an dirt road near the capital Sofia on Friday.

Prosecutors said the truck was abandoned near the village of Lokorsko after the driver and his companion found that many of the 52 migrants in the hidden compartments of the truck, which were isolated with foil, were dizzy and some had already died.

The truck driver and his companion were also charged over the deaths of the migrants, prosecutors said.

Despite strong and prolonged banging on the cabin, the driver refused to stop the truck earlier, the head of the National Investigative Service and deputy chief prosecutor Borislav Sarafov told reporters.

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

Thousands of people fleeing war and poverty in the Middle East, Africa and Asia make the journey and Bulgaria has been trying to cope with an increased inflow of migrants from neighbouring Turkey in the past year.

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", Sarafov said.

"The victims died slowly and painfully," he added.

"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, who were rushed to hospitals on Friday, remain in stable condition, officials said.

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