Police are on the hunt for a man in connection with the disappearance of a Vietnamese woman in a case that has been reclassified as a murder inquiry.

Officers on Wednesday said the man is in possession of a recognisance paper, which grants a person temporary stay in Hong Kong, and is believed to have spent time with the woman at a flat on Castle Peak Road where blood stains were found.

"We are investigating this case as a murder inquiry," superintendent Gar Kam-lam told a media briefing.

"Our preliminary investigations showed that the missing person and the suspect knew each other. We also do not rule out that this case could be related to a financial dispute."

Officers said the missing woman was never seen leaving the building after she entered with the man, while the man was seen coming in and out of the premises.

Police said the woman's family reported her missing on Saturday.

Investigations will continue, including at a landfill in Nim Wan in Tuen Mun, police added.

Officers added that a woman was arrested on suspicion of assisting offenders.