Executive councillor Lam Ching-choi agreed that the government had to catch up with the situation and roll out more prevention measures, said a South China Morning Post report.
His comments come against the backdrop of criticism that what authorities had done so far was too little, too late.
With an impending strike by various groups in the medical sector on Monday to press the government into full border closure with the mainland, Lam said he was more worried about the volume of Hong Kong residents crossing the border.
"If we do not reduce the number of those going north and carrying the virus back with them when they return, the risks of a local outbreak will be high," he said, citing figures indicating some 40,000 Hongkongers had entered Shenzhen on Friday, with 70,000 returning from across the border.
Comparatively, the numbers for mainlanders were around 9,000 and 13,000 respectively on the same day.
"Unless absolutely necessary, Hongkongers should not enter the mainland," he said. "Hongkongers have to be prepared that more measures would be rolled out in the short term."
Meanwhile, infectious diseases expert Professor Yuen Kwok-yung warned that if a current case in Hong Kong was proved to be a local transmission, it would be a very dangerous sign for the city.
Hong Kong confirmed its 14th case on Saturday evening, the South China Morning Post reported.
As of Sunday morning, the death toll due to the coronavirus outbreak was 304 in China, with 14,380 infected cases.
The Philippines has confirmed one death, making it the first country outside China to report a fatality.
Outside of China, the countries that have confirmed coronavirus cases are Thailand, Singapore, Japan, South Korea, Australia, Macao, Taiwan, Malaysia, the United Arab Emirates, the US, Germany, France, Vietnam, Canada, Italy, the UK, Russia, Cambodia, Finland, India, Nepal, Sri Lanka, the Philippines, Spain and Sweden.
The World Health Organisation has declared a global health emergency.
( With inputs from IANS )