HONG KONG - Hong Kong saw a staggering 48 new coronavirus infections on Friday, taking the tally to 256, as the city grapples with a huge number of residents returning from some of the worst-hit countries around the world.

The almost 25 percent spike in the number of confirmed cases was the highest single-day jump since the city reported its first infection in January, and nearly doubled the number on Wednesday, when 25 infections were reported.

Top infectious disease experts have warned residents against dropping their guard as the pandemic continues to take a heavy toll in Europe and the United States.

Chuang Shuk-kwan, head of the Communicable Disease Branch at the Centre for Health Protection, said 35 of the newly confirmed patients, aged from 4 to 69, had a record of overseas travel during the incubation period, including eight Hong Kong students who returned from countries like the United Kingdom, Canada and the US.

The youngest patient is a 4-year-old girl, whose father is believed to have contracted the disease during a trip to London. Another new local infection involved a taxi driver, who picks up passengers at Hong Kong International Airport, while the others are mostly close contacts of confirmed cases.

All but one of the 48 new infections involved Hong Kong residents. The only foreigner was an Australian traveler, who fell ill while on transit at the airport.

"It's likely the number of new infections will remain high in the next two weeks as more Hong Kong residents return from overseas," said Chuang, urging residents to continue practicing social distancing to avoid getting infected.

Gabriel Leung Cheuk-wai, dean of the Faculty of Medicine at the University of Hong Kong, warned that the city has entered the "most risky period" of a sustained community outbreak. But many people seem to have lowered their guard by joining outdoor activities and eating out, he said.

To make his point, he displayed two photos, with one showing more people gathering in Lan Kwai Fong, one of Hong Kong's most popular nightspots, on March 13 than the other taken earlier this month on the same spot. So far, six recently confirmed cases have been linked to the nightlife hub in Central.

"This is the worst moment to relax. It's absolutely critical when we have an influx of returnees since a few days ago, and continuing. We must be more vigilant," Leung said, noting that the number of imported cases from Europe has surpassed that from the mainland.

The scientist said the situation would be serious if one or two more clusters involving infected returnees emerge.

Ho Pak-leung, a leading microbiologist at HKU, compared the present situation to "being at war", with at least 200 imported cases expected with the huge number of Hong Kong residents coming back from coronavirus-ravaged areas in the next two to three weeks. He urged residents to stop visiting pubs and gyms.

Ho said self-quarantine in a limited home environment poses a very big risk for family members of an underlying patient as it could spark a major outbreak in the community. He suggested that people arriving in Hong Kong be housed in hotels.

Ho also said public hospitals may struggle to cope with the growing number of patients in a short time with only about 50 beds available in intensive care units.

As of Friday, up to 47 percent of 954 isolation beds in public hospitals had been occupied, with more than 70 percent of 507 isolation wards in use.

The Hospital Authority said it plans to upgrade 400 beds for isolating patients by the end of this month, but may have to cut more non-emergency services if the situation worsens.