Police have intensified their hunt for a suspected South Asian serial killer after a fourth prostitute was killed in just three days. Terrified sex workers in the city have been warned to stay alert. Commissioner of Police Tang King-shing expressed serious concern about the series of killings in such a short period and said frontline officers had stepped up patrols. Police are hunting for a strongly built man of South Asian appearance but have yet to make any arrests. The latest victim was Tam Siu-fong, 27, whose naked body was found in the toilet of the first-floor flat she used as a one-woman brothel in Electric Road, North Point, at 2.23pm yesterday. The first prostitute was found dead in Yuen Long early on Saturday. The second one was found unconscious on Saturday evening in Tai Po and declared dead on Sunday morning, and the third prostitute was killed on Saturday night in Tai Po and her body found early yesterday morning. An initial examination showed injuries to Tam's forehead and lower jaw, and that she had been dead for some time, a police source said. Before Tam's murder was discovered, Senior Superintendent Steve Li Wing-hong, of New Territories North regional crime unit, said there were similarities in the first three cases: all were sex workers, all were killed in their workplaces - one-woman brothels - and their flats had been ransacked. He said police did not rule out the possibility that a serial killer was at work. 'We are keeping an open mind and looking into all possible clues. We will spare no efforts to investigate the cases,' Mr Li said. All sex workers who run one-woman brothels were living in fear, said Elaine Lam Yee-ling, spokeswoman for sex workers' group Zi Teng. 'They are very worried and yet they cannot stop working, as they have families to feed,' she said. Zi Teng received more than 100 inquiries about the serial killings before noon yesterday. With the four deaths, 14 people have been murdered in Hong Kong this year, compared with three in the first quarter of last year.