Three victims, all in their 30s, were found in their ransacked apartments in two districts in the northern New Territories. A fourth was found on Hong Kong island on Monday.

The first two women had been robbed and strangled, one with a hose, while the third was certified dead with no apparent injuries early on Monday.

'We are adopting a very open-minded approach, we will rigorously pursue all lines of inquiries,' said Mr Steve Li, a police superintendent, when asked if a serial killer may have been responsible.

'We can't rule out the possibility that either one or more persons carried out these crimes.'

Murders of prostitutes are rare in Hong Kong. In 2003 and 2004, a retired Chinese soldier killed two sex workers during a visit to the former British colony.

Zi Teng, a help group for sex workers, said the three victims were among 1,800 women who advertised in local newspapers for services rendered in the privacy of their own apartments.

While there are no official statistics on the size of Hong Kong's sex industry, experts say there are at least 100,000 women working at any given time, consisting mostly of a transient pool of women from China's impoverished inland provinces.

Local TV footage showed police on Monday combing vice dens and urging sex workers to be vigilant. 'With such grave crimes having taken place, we are taking them very seriously and are actively investigating,' said police commissioner Tang King-shing. -- REUTERS