An unidentified gunman is targeting motorists on highways near the southwestern port of Rotterdam, where 37 cars have had windows shot out since August 7, Dutch police said Wednesday.

"The gunman is either operating from inside a car, or from the side of the road," spokesman Frans Zuiderhoek told AFP, adding nobody had been wounded.

Police found no bullets in the damaged cars, which all had rear windows shot out, suggesting the "Highway Shooter" as the suspect has now been dubbed in the local press, was using a pellet-gun, Zuiderhoek said.

"Up to now, there fortunately has been no serious accident, but shooting at cars, even with a pellet-gun, can lead to very dangerous situations," police said in a press release.

They offered a 10,000 euro reward for information leading to the gunman's arrest.

"We do not know whether it's a person or persons," Zuiderhoek admitted. So far the investigation has not pinpointed the type of bullet used either.

"We have received more than 300 calls, none of them leading to the shooter," he added.

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