Hong Kong on Saturday declared a new coronavirus outbreak as an "emergency" - the city's highest warning tier - as authorities ramped up measures to reduce the risk of further infections.The announcement came as city leader Carrie Lam faced criticism in some quarters over her administration's response to the crisis. Of the five people who have tested positive for the virus in Hong Kong so far, four arrived via a newly built high-speed train terminal which connects with the mainland.That led to calls from some medical experts and politicians to limit, or even halt, arrivals from China, the epicentre of the outbreak with 41 people dead.Lam held emergency meetings with health officials on Saturday morning after returning from Davos."Today I declare the lifting of the response level to emergency," she told reporters.Schools and universities, which are currently on a Lunar New Year break, would remain closed until February 17, Lam said.All mainland arrivals to Hong Kong would now need to sign health declaration forms, she added, while public events, including a new year gala and next month's marathon, would also be called off. "We haven't seen serious and widespread infections (in Hong Kong), but we are taking this seriously and we hope to be ahead of the epidemic," Lam said.Ho Pak Leung, a microbiologist at the University of Hong Kong, said closing the mainland border would be the "single most effective measure" in containing the virus.Transport links with the mainland should be reopened "when the epidemic is controlled, when HK officials are awake, when there are enough masks and hand rubs for all Hong Kong citizens", he wrote on his Facebook page.Legacy of distrustHong Kong has a recent experience of deadly viral outbreaks.Nearly 300 people were killed by SARS in 2003, a tragedy that left a profound psychological impact on one of the most densely populated places on earth.The city's ability to combat the crisis was hampered by moves in mainland China to cover up and play down the outbreak, leaving a lasting legacy of distrust among many Hong Kongers.Animosity towards the mainland has intensified in recent years as Beijing tightens political control over the semi-autonomous territory. The outbreak also comes at a sensitive time for Lam, who has record low approval ratings after seven months of pro-democracy protests."We must stand united so that we can prevent and control the disease," she said, in a nod to the political unrest.The often violent protests have battered Hong Kong's reputation for stability and helped tip it into recession, with the recent virus outbreak compounding the city's economic woes. Hospitals are already struggling with the winter flu season, but officials are isolating anyone with a history of travel to central China and those exhibiting respiratory tract infections that look similar to the virus.So far, some 300 people have been tested and monitored for the virus. Quarantine centres have been set up in remote holiday parks for anyone found to have come into close contact with people who tested positive.On Saturday, officials announced that a newly built, but still-empty, public housing block would be used for medical staff on the frontline who did not want to risk returning to their families.