Three young Syrian children found in critical condition inside a minivan in Austria have disappeared from hospital.

The two five-year-old girls and one six-year-old boy were suffering from dehydration when police stopped the vehicle containing 26 refugees on Friday.

Officers have now said the children and their families have gone missing from a hospital in the town of Braunau am Inn, where they were being treated.

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Rescued: Inspector Jan Gieber shows the inside of the van which contained 26 refugees when it was stopped by police

The children were suffering from dehydration when police stopped a minivan (pictured) containing 26 refugees

When it was stopped in the small town of St. Peter am Hart, close to the German border, the van contained 26 refugees from Syria, Afghanistan and Bangladesh, according to local press.

Police arrested the 29-year-old Romanian driver.

'It was a very close call,' David Furtner from Austria's police force said, explaining that the children had been found dizzy due to dehydration in the hot and sticky lorry.

'Medical staff told us they would not have made it much longer,' he said, revealing the children and their parents were taken to the hospital in the nearby town of Braunau am Inn.

The three children and their families have disappeared and authorities believe they may have tried to cross the border into Germany rather than being deported back to Hungary.

'We have just found out that they left hospital yesterday (Saturday) with their parents,' Mr David Furtner said.

'Then they vanished, and we think they went to Germany.'

The Austria Press Agency on Sunday quoted a doctor at the hospital in Braunau am Inn saying that although the children would have been kept in, 'from a medical point they were not in danger any more'.

The news of the discovery of the survivors in the minivan came after three Bulgarians and one Afghan citizen suspected of transporting 71 refugees, who were found dead in a truck in Austria on Thursday, appeared in court in Hungary.

A prosecution spokesman told journalists the four faced human trafficking charges involving torture and targeting financial gain.

The 71 victims included eight women and four children – the youngest a girl between one and two years old and three boys aged eight to ten.

Migrants jump through the border fence between Serbia into Hungary close to the village of Roszke on Sunday

A Hungarian Army truck drives past a group of Afghan men who crossed from nearby Serbia and sleeping on a bicycle path in Morahalom, Hungary

Migrants line up to board a bus heading for a transit centre for migrants at the border between Hungary and Serbia

Many of the bodies had decomposed, suggesting they had been dead for several days in the back of the air-tight refrigerated lorry that usually carried frozen chicken.

Prosecutors said they wanted the three Bulgarians and one Afghan remanded in custody due to the 'exceptional nature of the crime, the subsequent deaths of the smuggled persons and the perpetration of the criminal act of people-smuggling in a businesslike manner'.

The four claimed they were innocent, but were remanded in custody until September 29.

On Sunday, Hungarian police said a fifth suspected human trafficker - a Bulgarian national - had been arrested over the discovery.

The five people now in custody in Hungary are believed by police to be low-ranking members of one of the numerous and often unscrupulous human-trafficking gangs.

When the lorry was discovered, police thought the parked vehicle had broken down with a mechanical problem.

However, when they approached the vehicle, they realised the back door had been secured shut with wires.

Its refrigeration system showed no signs of having been switched on and there were no vents to allow fresh air inside.The victims had been wearing light summer clothes.

Migrants who have been picked up in Austria near the border with Hungary, line up at a makeshift camp

A prosecution spokesman told journalists the four faced human trafficking charges involving torture and targeting financial gain

Prosecutors said they wanted the three Bulgarians and one Afghan remanded in custody due to the 'exceptional nature of the crime'

The 71 victims included eight women and four children – the youngest a girl between one and two years old and three boys aged eight to ten

Officers recalled seeing 'blood dripping' from the vehicle and 'noticed the smell of dead bodies,' Hans Peter Doskozil, chief of police in Burgenland province, said.

Interior Minister Johanna Mikl-Leitner said the tragedy 'should serve as a wake-up call... for joint European action' in dealing with the torrent of migrants flocking to Europe.

Melissa Fleming, spokeswoman for the U.N. refugee agency in Geneva called the tragedy 'absolutely shocking.'

'We believe this underscores the ruthlessness of people smugglers who have expanded their business from the Mediterranean Sea to the highways of Europe.

It shows they have absolutely no regard for human life, and that they are only after profit,' she said.

'It also shows the desperation of people seeking protection or a new life in Europe, and their only means is to submit themselves to these criminals.'

It is believed the migrants boarded the truck in Serbia and were transported across the Hungarian border and through into Austria.

It is believed that the migrants boarded the truck in Serbia and were transported across the Hungarian border and through into Austria

The truck apparently used to belong to the Slovak chicken meat company Hyza, part of the Agrofert Holding, which is owned by Czech Finance Minister Andrej Babis.

Agrofert Holding, in a statement, said they had sold the truck in 2014.

The new owners did not remove the truck's logos as required and Hyza had nothing to do with the truck now, the company said.

On one side of the truck was the slogan 'Honest chicken,' while writing on the back read 'I taste so good because they feed me so well.'

Austrian Interior Minister Johanna Mikl-Leitner said that the European Union had to tighten immigration inspections to prevent future similar disasters.

The deaths caused an international outcry as hundreds of thousands of migrants continue to flee conflict zones in search of sanctity in the EU.

People light candles at Keleti railway station in Budapest, Hungary, during a memorial for the 71 migrants who were found dead in a vehicle on an Austrian highway

Migrants attend a candle light ceremony at a Budapest railway station in memory of 71 refugees who died in a lorry

On Sunday, people were pictured climbing through a barbed-wire fence Hungary has installed along its southern border.

In the sweltering heat, families with babies and small kids could be seen sitting on blankets and in makeshift tents in a transit zone in front a station in central Budapest station after crossing the border.

Children played volleyball, oblivious to a silent ceremony on the steps leading to the train station in memory of the 71 migrants who died on Thursday.