The man’s mother and father, two aunts and three cousins were the others infected. Their ages range from 22 to 68.

The other case confirmed earlier was a 70-year-old man who had not travelled out of Hong Kong since January 9, spending most of his time at home.

With the 10 new cases, the number of people infected in the city jumped by more than a third to 36, heightening fears of a community outbreak.

The development came as health authorities warned of “major difficulties” in tracing possible virus carriers because some might only show mild flu-like symptoms at an early stage.

In light of the increasing number of cases, the Hospital Authority, which runs the city’s public health facilities, also said it would drastically adjust non-urgent services in the coming four weeks.

“We’re facing major difficulties in isolating the suspected cases and tracing those who had close contact with the confirmed patients,” Dr Chuang Shuk-kwan, head of the communicable disease branch of the Centre for Health Protection, said, adding that it was because some people would only show mild symptoms and thus it was hard to tell who might have the virus.

Fake news rising along with the number of coronavirus cases

Nine of the new cases had been to a family gathering on January 26 at the Lento Party Room in Kwun Tong. Nineteen people had joined the dinner, including two relatives from mainland China who left the city at the end of last month.

The two sought medical attention across the border. One did not have the coronavirus while test results were pending for the other.

“I suggest the public cuts down on these gatherings. If they are necessary, try to reduce the time spent together,” said Chuang, who also urged citizens not to share chopsticks with those they dine with.

All of the 1,800 crew members, who possibly had contact with eight passengers infected with the new virus on a previous trip, tested negative for the disease.

On whether Hong Kong could stop the spread of the virus in the community, Chuang said it depended on how many virus carriers there were who showed little or no symptoms.

Chinese diplomat pushes back against virus ‘rumours’ from Republican senator

“If there are many people who have no symptoms or only mild symptoms, and they have infected many others, then there isn’t much we can do to stop the spread,” she said. “We will do all we can.”

Two medical sources, meanwhile, said a 69-year-old man with diabetes had also tested positive for the coronavirus, possibly raising the total tally further. He remained in critical condition on Sunday at Pamela Youde Nethersole Eastern Hospital’s intensive care unit. He travelled to the mainland more than a month ago, the sources said.

Infectious diseases expert Dr Joseph Tsang Kay-yan warned: “Infection among family members will continue to increase because we have not closed the border as stringently as Macau.

“What the Centre for Health Protection said [on Sunday] is a clear signal that citizens have to take care of their own health, by reducing social contact and care about personal hygiene.”

In Macau, anyone from Hubei – epicentre of the outbreak – and those who had visited the province in the past 14 days must present a medical certificate proving they did not have relevant symptoms before entering the city. Casinos, along with 18 other types of premises including bars, cinemas and massage parlours, were also forced to close for 15 days earlier this month.

The number of people entering Hong Kong dropped sharply as a 14-day mandatory quarantine scheme to tackle the coronavirus outbreak took effect on Saturday.

Latest coronavirus deaths in China push death toll past Sars

On that day, only 23,399 people entered the city through the airport, Hong Kong-Zhuhai-Macau Bridge and Shenzhen Bay Port, the three control points that remain open, down from 95,982 on Friday. Of these, only 1,430 came through land crossings on Saturday from Macau and the mainland.

From Saturday to 7pm on Sunday, 918 people were put under mandatory quarantine. They included 814 Hong Kong residents.

Those who enter from Macau are not required to go into quarantine.

While the numbers have dropped, critics have questioned the nature of the quarantine, as it relies on people to stay at home or in their hotels, though police will conduct random checks on them.

Professor David Hui Shu-cheong, a respiratory medicine expert from Chinese University, said it was too early to conclude that Hong Kong had edged closer to a community outbreak with the new cases.

“It still originated from a family gathering where people had close contact,” Hui said.

Lento Party Room said in a social media post it had closed its doors for now and that none of its employees had shown any symptoms of the infection. Staff would isolate themselves at home for 14 days. On Sunday afternoon, three health officers in full protective gear entered the venue to assess the situation.

Not just China: travellers avoiding Singapore, Japan amid virus outbreak

Meanwhile, angry residents held small protests in Tai Po, Sai Kung and Fo Tan against the government’s plans to set up quarantine centres or designated clinics to treat patients with mild fever and respiratory illness.

In Sai Kung, dozens took part in the protest. Police raised a blue flag warning the protesters they were taking part in an unauthorised assembly before dispersing them. Officers chased them into the Lakeside Garden housing estate and one subdued man was seen bleeding from his head and hand.

Three camps set up for quarantine purposes had already reached their capacity, and preparations were being made to convert the Heritage Lodge of the Jao Tsung-I Academy in Lai Chi Kok into a fourth centre, the government said.

Hours before the protests, Hong Kong’s No 2 official Matthew Cheung Kin-chung and Secretary for Food and Health Sophia Chan Siu-chee both sought to assure the public that the city’s food supply was sufficient, amid panic buying of essentials after signs of a community outbreak.

They said there was no need to worry about rice stocks and other necessities, and that such goods were not affected by the new quarantine measures.

“The government has already talked to major food suppliers, and they have publicly said that supplies are stable, so citizens should not worry,” Cheung wrote on his weekly blog.

Speaking separately on a Commercial Radio programme, health minister Chan urged residents not to rush to buy or stockpile rice. She said there were 25 million kilograms of rice in stock, sufficient to feed the city for about a month.

“These quarantine sites are very safe, and the distance from residential areas is more than sufficient to stop the virus from spreading,” Lam said after a television programme. “We have to have faith in science and these experts.”