Forensic police officers inspect a parked truck in which up to 50 migrants were found dead, on a motorway near Parndorf, Austria August 27, 2015.[Photo/Agencies]

PARNDORF, Austria - As many as 50 refugees were found dead in a parked lorry in Austria near the Hungarian border on Thursday, and German Chancellor Angela Merkel said the discovery had shaken European leaders discussing the migrant crisis at a Balkans summit.

Police made the grisly discovery in the 7.5-tonne lorry stopped on the A4 motorway near the town of Parndorf, apparently since Wednesday, Hans Peter Doskozil, police chief in the province of Burgenland, told a news conference.

He said he could not put an exact figure on the number of victims, whose bodies had begun to decompose. "We can assume that it could be 20 people who died. It could also be 40, it could be 50 people," he said.

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

Tens of thousands of people, mainly from Africa and the Middle East, have put to sea this year in the hope of reaching Europe, often dangerously packed into small vessels that were never designed to cross the Mediterranean.

Those who make it ashore and others travelling by land have increasingly tried to make their way north via the Balkans, causing tension among countries along the route.

Hungary plans to reinforce its southern border with helicopters and mounted police, and is considering using the army as record numbers of migrants passed through coils of razor-wire into Europe.

Investigations were underway in Austria and Hungary after the bodies were discovered. The truck had Hungarian number plates, a Hungarian official said.

Janos Lazar, Prime Minister Viktor Orban's chief of staff, said a Romanian citizen had registered the number plate in the eastern Hungarian town of Kecskemet.

Police limited the motorway to one lane while forensic experts checked over the lorry parked on the hard shoulder.

Austrian Chancellor Werner Faymann told the summit: "The refugees who died today wanted to save their own lives by fleeing, but instead lost their lives at the hands of traffickers. It shows once again how necessary it is to save human lives by fighting criminal traffickers. It shows that we must take responsibility and give asylum to those people who are fleeing."