Up to 50 people, presumed to be asylum-seekers, have been found dead in the back of a lorry in Austria.

Officials referred to the dead as migrants, and local media reports suggested they died of "suffocation" after being trapped in the lorry's trailer. A police manhunt has been launched to find the driver, who is suspected of smuggling the victims into the country.

The bodies were found in the back of a refrigerated food delivery truck, which had been parked on the hard shoulder of the A4 road, Austria's "Eastern Motorway", near the town of Parndorf.

A spokesperson for the Austrian interior ministry confirmed to the Reuters news agency that the tractor trailer had been found, but was unable to provide a definitive death toll. "Unfortunately there are many dead people in there," he said.

Police stand in front of a truck parked on the shoulder of the highway A4 near Parndorf south of Vienna (AP)

At least 20 migrants were found dead in the truck parked on the Austrian highway leading from the Hungarian border, police said. (AP)

'Today is a dark day'

Austrian police said the vehicle had been left abandoned at the site since Wednesday, but was only searched by officers on Thursday morning after it was repored by local workers.

They said there were between 20 and 50 bodies inside, and that they had been dead for some time and started to decompose.

Tweeting from the scene, Channel 4 News' Lindsey Hilsum said there was a "terrible smell of death as we passed".

Austria's interior minister Johanna Mikl-Leitner addressed the discovery of the lorry at a press conference in Eisenstadt, according to Krone.

She hit out at the "despicable methods" of migrant smugglers, saying they "belong behind bars", and said: "Today is a dark day."

Macedonia/Greece Border Crisis Show all 10 1 /10 Macedonia/Greece Border Crisis Macedonia/Greece Border Crisis Macedonia Border Crisis Migrants attempt to pass the Greek-Macedonian border guarded by Macedonian police near the town of Idomeni, northern Greece AFP/Getty Macedonia/Greece Border Crisis Macedonian Border Crisis A migrant reacts as he carries a child during clashes with Macedonian police at the Greek-Macedonian border Reuters Macedonia/Greece Border Crisis Macedonia Border Crisis A Macedonian policeman armed in riot gear clashes with a migrant girl, police have reinforced control at the border with Greece in a bid to stop the influx of migrants, but a few hundred Syrians managed to cross the frontier overnight AFP/Getty Macedonia/Greece Border Crisis Macedonia Border Crisis Macedonian special policemen guard the border as more than a thousand immigrants wait at the border line Reuters Macedonia/Greece Border Crisis Macedonia Border Crisis Migrant men help a fellow migrant man holding a boy as they are stuck between Macedonian riot police officers and migrants during a clash near the border train station of Idomeni, AFP/Getty Macedonia/Greece Border Crisis Macedonia Border Crisis A barbed wire fence lines the border of Macedonia and Greece near the Gevegelija Railway station, Macedonian special policemen are guarding the border as more than a thousand immigrants wait at the border line of Macedonia and Greece Reuters Macedonia/Greece Border Crisis Macedonia Border Crisis Macedonian Police stand firm at the Greek-Macedonian border in a bid to stop the influx of migrants AFP/Getty Macedonia/Greece Border Crisis Macedonia Border Crisis Government of Macedonia has declared the state of emergency in the region of country's southern and northern border and in accordance with the law to open a possibility for appropriate engagement of the army of Macedonia AFP/Getty Macedonia/Greece Border Crisis Macedonia Border Crisis Macedonian police drove back crowds of migrants and refugees trying to enter from Greece on Friday after a night spent stranded in no-man's land by an emergency decree effectively sealing the Macedonian frontier. Reuters Macedonia/Greece Border Crisis Macedonia Border Crisis A migrants woman with childrens wait to pass the Greek-Macedonian border, guarded by Macedonian police near the town of Idomeni, northern Greece AFP/Getty

Responding to the discovery of the bodies, Amnesty International's deputy director for Europe, Gauri van Gulik, said: “People dying in their dozens - whether crammed into a truck or a ship - en route to seek safety or better lives is a tragic indictment of Europe’s failures to provide alternative routes.

“What’s urgently needed is what Amnesty International has been saying for years: Europe has to step up and provide protection to more, share responsibility better and show solidarity to other countries and to those most in need.”

Earlier on Thursday, Austrian police said they had stopped three lorries suspected of carrying migrants. They included one van which had taken 34 refugees, including 10 children, across the border into Austria.

The incident comes as Europe faced a continent-wide migrant crisis, with neighbouring Hungary reporting 3,241 illegal entry attempts - a record number - on Wednesday.

Hungary is close to completing a razor-wire fence spanning the full 110-mile length of its border with Syria, prompting a race for migrants to reach the effective start of the Schengen Area of free movement within the EU.