Theft cases on airplanes have risen sharply this year, especially during travel peak seasons, data from the Hong Kong police force showed.

Police received 67 reports of luggage thefts on airplanes in the 10 months to October, up 40 percent from 48 cases for the whole of last year, Ming Pao Daily reported.

Theft is usually committed during flight, when flight attendants are busy distributing meals, when the cabin light is dimmed, or when passengers leave their seats, the newspaper said.

Police warned passengers to make sure their luggage are locked or they carry their valuables inside hand-held bags, especially during holidays when thieves are most active.

Ip Chun-wing, detective chief inspector at the Airport District, said thieves mainly come from the mainland and southeastern countries, who travel quite frequently but only stay briefly in each place.

Investigation showed that thieves often work in groups.

They take different seats on the plane and pass around the stolen goods, Ip said.

They would store or open their luggage in the overhead bin but actually search for targets such as unlocked bags or portable goods, he said.

Once stolen, the items are passed on to another member of the gang.

In order to arrest and prosecute suspected thieves, investigators need the help of witnesses and cabin crew members, Ip said.

Those found guilty of theft aboard Hong Kong-registered flights could face up to 10 years' imprisonment, but the punishment for cases on other flights is normally about six months in jail, he added.