Travelers are being advised to not place valuables in the luggage compartments of the buses amid suspicions that small thieves are being hidden inside large sports bags.

It is unknown whether children or very small adults are used for the robberies but police said the thieves emerge from stowed bags mid-journey before climbing back inside with their loot.

"It is very possible that a small person is being placed in a bag in order to search through the other bags. We have recently received several reports of thefts, including on the Vasteras to Stockholm line," said Swebus spokeswoman Pia Kravall.

Police say a number of similar luggage compartment robberies have also popped up in other parts of the country.