As many as 50 migrants have been found dead inside a parked truck on a highway in Austria, according to police.

The 7.5-tonne poultry truck was found sitting on the side of the eastern A4 highway near the Hungarian border, police spokesman Hans Peter Doskozil said.

• ‘Nope, nope, nope’ to migrants
• Mass graves of suspected migrants found
• Brit PM under fire for ‘dehumanising migrants’

Fluids from the bodies were found seeping out the truck’s back door.

“One can maybe assume that the deaths occurred one-and-a-half to two days ago,” Mr Doskozil said.

Austrian police said it may take a day to confirm the body count.

Austrian interior minister Johanna Mikl-Leitner called it “a dark day” and underscored Vienna’s demand that the European Union adopt a way to distribute the increasing wave of migrants fairly so that they would not have to turn to human traffickers.

“This tragedy affects us all deeply,” Ms Mikl-Leitner said.

“Human traffickers are criminals. Anyone still thinking that they’re kind helpers cannot be helped.”

It was not immediately clear how or when the migrants died.

The truck reportedly had Hungarian number plates. Investigations were underway in Austria and Hungary after the discovery.

The gruesome find came as German chancellor Angela Merkel and Balkan leaders met in Vienna to discuss a solution to the biggest migration crisis to hit Europe since World War II.

“We are of course all shaken by the appalling news,” Ms Merkel said.

“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.”

This year has seen record numbers of people trying to reach the European Union by sea and land as they flee conflict in Africa, Asia and the Middle East.

“This is a signal on the European level to act as quickly as possible here,” Ms Merkel said, adding that controls on international trains and border areas had been stepped up.

Ahead of the conference, Austria’s foreign minister Sebastian Kurz warned that his country would consider introducing tougher anti-migration measures including “much tighter border controls”, if the EU failed to come up with a unified response.

“Austria has more migrants than Italy and Greece combined … so we shouldn’t pretend that only Italy and Greece are affected,” he said.