HONG KONG -- Four local cases of dengue fever (DF) have been registered in Hong Kong in a short period of time this month, which is "unusual" in the city, the Hong Kong Special Administrative Region (HKSAR) government's Centre for Health Protection (CHP) warned.

The four DF cases, confirmed Tuesday by the CHP, involve a 17-year-old male and three seniors aged 76 to 84.

The four patients, living in different areas of Hong Kong, fell ill earlier this month and sought treatment at different hospitals. All of them tested positive for the Dengue virus, and are in a stable condition, the CHP said Tuesday night.

The CHP Controller Wong Ka-hing said a few local cases were recorded in Hong Kong in the past few years, and it is unusual to find four local cases in such a short period of time.

The CHP is very concerned about the situation, he said, adding that he does not rule out the possibility that more cases will be found.

Wong warned that the city must remain cautious to avoid local transmission.

The CHP said it would conduct site visits to the patients' homes and carry out active case finding starting Wednesday. Health talks will also be arranged near the residences.

It has also informed Guangdong province and Macao health authorities about the cases and will issue letters to local doctors and hospitals to update them on the latest situation.

The HKSAR government's Food & Environmental Hygiene Department will step up control measures and conduct mosquito surveys in areas visited by the patients.

Other than the four local cases, as of Aug 9, 55 imported cases have been recorded in 2018.