Austria has tightened controls on motorways near the Hungarian border, where an abandoned lorry was found last week containing 71 dead migrants, four of them children.
The measure, which was put in place late on Sunday, involves increased controls on traffic in Burgenland state which will be carried out in close collaboration with the Hungarian, Slovakian and German authorities.
"The main aim is to target smuggling gangs. What is happening here are controls conducted by traffic police and security forces -- these are not border controls," said police spokesman Helmut Marban.
As part of the clampdown on human traffickers, police are stopping every truck, van and car in an effort to catch those trying to make money from people fleeing war and persecution.
Shortly after the checks began at 1830 GMT, officers discovered 12 migrants - nine adults and three children - crammed inside a minivan carrying a French number plate
The group was believed to be predominantly from Syria, media reports said.
The driver, whose nationality was not immediately clear, was arrested.
The new security checks caused a huge build up of traffic overnight, and by Monday morning, there were tailbacks of up to 30 kilometres (more than 18 miles) along the M1 motorway connecting Budapest to Vienna, Hungarian media reported.
Many migrants picked up in Austria have already trekked on foot through four or five countries before they reach EU member Hungary, where smugglers pack them into vehicles without food or water, promising to transport them to richer European states like Germany or Sweden.
So far this year, the Hungarian government has reported more than 140,000 migrants entering the country from Serbia.
Since Hungary is in the visa-free Schengen zone, onwards travel has been relatively easy.
But Austria's reinforced border units and tightened security checks are likely to make the process of entering the country illegally much harder.
This year alone, police in Upper Austria have arrested 93 human traffickers transporting a total of 1,630 migrants.