German Chancellor Angela Merkel says she's shaken by the appalling news that up to 50 refugees have been found dead in a lorry in Austria.

It is understood the truck, which had Hungarian number plates, was parked at the side of the motorway for an extended period of time.

Austrian newspaper Krone reported that early indications were that the refugees had died of suffocation.

Chancellor Merkel spoke to reporters in Vienna, where the EU's holding a summit on the refugee crisis: