Prosecutors at an appeal court in Szeged, 160 kilometres (100 miles) south of the Hungarian capital Budapest, successfully argued that the initial 25-year jail terms delivered last June were too lenient.

The victims — from Syria, Iraq and Afghanistan — were among hundreds of thousands of desperate people fleeing war and poverty in the Middle East and elsewhere in 2015, triggering Europe's worst migrant crisis since World War II.

Hungary took over the proceedings from Vienna after it emerged that the migrants had suffocated on Hungarian soil.

The sentences of gang ringleader Afghan national Samsoor Lahoo and two Bulgarian accomplices, including the truck's driver, were increased to life without parole.

A fourth man, also Bulgarian, was also given a life sentence and must serve a minimum term of 30 years.

Ten other suspects were also found guilty over the deaths and handed prison sentences of between four and eight years.

Hungarian prosecutors said the ring was a professional trafficking network with more than 15 vehicles used to transport refugees trekking up from Greece along the western Balkans to Europe.

The Budapest-based gang smuggled more than 1,100 people from Hungary into Austria from February 2015, charging up to 1,500 euros ($1,760) a head.

The bodies of 59 men, eight women and four children — including a baby girl — were already in an advanced state of decomposition when they were discovered in an abandoned poultry refrigerator lorry on August 27, 2017.

Investigations showed they had been dead for two days, suffocating shortly after being picked up in Hungary, then a key transit country on the Balkan migrant trail.

Lahoo and the other accused denied knowing that the migrants were dying in the back of the truck. But evidence presented to the court indicated they had been aware of what was happening.