They say a patrol chased the van, which then swerved off the road due to high speed early Wednesday. Several people have been hospitalized, one with serious injuries.
Police say the van had Austrian license plates and a Ukrainian driver who tried to escape by a crossing a river but stopped after a police warning.
The incident illustrates dangers migrants face as they try to cross borders illegally in an attempt to reach wealthy European nations.
Several thousand have been stuck in Serbia waiting for a chance to move on.