Hong Kong - A Hong Kong teenager who robbed seven prostitutes at knifepoint, in some cases after having had sex with them, was jailed for six years Thursday.

Li Ka-chun, 18, carried out a series of robberies in June 2008 in inner-city flats where the prostitutes worked by themselves.

He admitted seven robberies and was jailed at a hearing in Hong Kong's High Court Thursday by judge Gareth Lugar-Mawson who told him his crimes were very serious.

The case comes just two months after a jobless Hong Kong man who repeatedly slept with prostitutes and then tied them up and robbed them was jailed for nine years.

Chong Kwonk-hung, 24, admitted stealing hundreds of dollars from four prostitutes he slept with between October 2006 and June last year.

Pressure groups representing sex workers have been calling for a change to a Hong Kong law which allows women to operate as prostitutes only if they work in a flat on their own.

In 2008, four prostitutes were murdered in the space of three days in their flats. A suspect was later arrested and charged with three of the killings. (dpa)