"Unfortunately there are many dead people in there," the spokesperson told Reuters. He described the people as refugees, but said he did not have an exact number of those found in the lorry.

Police cars round truck in which refugees died at side of #Austrian motorway. Travelling fm Serbia, they suffocated.

Meanwhile, Austrian police say the number of those dead could range from 20 to 50.

"The level of information is currently very poor. One can imagine how the condition of these people is. Therefore we can not yet say how many dead there are also," National Police Director Hanspeter Doskozil said.

The truck, which has Hungarian license plates, is believed to have been abandoned for several days.

Hungarian Prime Minister Viktor Orbán's chief of staff, János Lázár, said that Hungarian law enforcement are working with Austrian police to track down the driver.

Police received an alert about the vehicle around 11:30 a.m. local time on Thursday. When officers arrived at the scene, they discovered putrefaction liquid at the rear of the truck.

Interior Minister Johanna Mikl-Leitner said during a press conference in Eisenstadt that “today is a dark day.”

She spoke of the "despicable methods" of migrant smugglers, saying they "belong behind bars."

Earlier on Thursday, Austrian police said they had stopped three vehicles suspected of carrying refugees. Included in that number was a van which had taken 34 migrants, including 10 children, into the country.

Just drove past truck on A4 in Austria with 50 dead refugees inside. Terrible smell of death as we passed.

It comes amid a worsening migrant crisis in Europe, with tens of thousands of refugees from the Middle East and Africa – primarily Syria – trying to make their way to Europe's borderless Schengen zone.

More than 28,300 people applied for refugee protection in Austria in the first half of 2015.

Meanwhile, the Hungarian prime minister's chief of staff says the number of migrants trying to reach western Europe through Hungary could reach 300,000 by the end of the year.

Meanwhile, German Chancellor Angela Merkel said everyone is "shaken" by the news of the migrant deaths in Austria.

"We are of course all shaken by the appalling news," Merkel told a news conference at a summit on the West Balkans in Vienna.

"This reminds us that we must tackle quickly the issue of immigration and in a European spirit – that means in a spirit of solidarity – and to find solutions."

Merkel is meeting with Balkan leaders in Vienna to discuss Europe's biggest migrant crisis since World War II.

Countries taking part include Macedonia and Serbia, both of which are among the hardest hit, as they are transit nations for those trying to reach western and northern Europe.