Hong Kong issued a "red alert" advisory on Tuesday against non-essential travel to South Korea, where eight new cases of Middle East Respiratory Syndrome (MERS) were reported bringing the total to 95 with seven fatalities.
The number of new South Korean cases was a sharp drop from 23 on Monday, but the number of schools closed grew to 2,208, including 20 universities.
"At this stage, to issue a clear message is something the Hong Kong government thinks is necessary," Hong Kong's number two official, Carrie Lam, told reporters just before the travel warning was posted.
A red alert, the second-highest outbound travel advisory on a three-point scale, is defined as a "significant threat" according to the Hong Kong government, and means people should "adjust travel plans" and "avoid non-essential travel".
On Monday, Hong Kong upgraded its response to the outbreak in South Korea to "serious".
Nam Kyung-pil, governor of Gyeonggi province, which surrounds the South Korean capital, Seoul, said 32 of its large general hospitals have joined the campaign to fight the outbreak by offering to take in anyone who is showing MERS symptoms.
"We are fighting two wars; The war against the disease and the war against fear," Nam said.
The head of the Korean Hospital Association, who accompanied the country's deputy prime minister on a Tuesday visit to a Daejeon hospital where MERS patients were being treated, criticised the government for poor communication.
"The hospitals that did not receive information on patients have been wounded deeply," Park Sang-geun said during an open meeting.
It was only on Sunday that South Korean officials released the names of all the health facilities where MERS victims had been treated or visited, which now number 35.
Tourists cancel trips
The World Health Organization (WHO) began work on a joint mission with South Korean doctors and officials to review the country's response and analyse the virus.
The WHO has not recommended any curb on travel, but thousands of tourists have cancelled plans to visit South Korea.
The Travel Industry Council of Hong Kong has cancelled all tours to South Korea thate were scheduled to set off between now and June 30, excluding cruises, with 10,000 to 12,000 travellers to be affected, the city's public broadcaster reported.
South Korea's response to the outbreak has been in line with WHO guidelines, Alison Clements-Hunt, a spokeswoman for the WHO director-general's office, told Reuters in Manila.
"There are still a lot of people in isolation ... we would expect some of those cases may develop MERS but hopefully this will start to tail off as the outbreak control measures and contact tracing kicks in," she said.
South Korean President Park Geun-hye has called for an all-out national effort to eradicate the outbreak, which has been spreading since a 68-year-old businessman brought it home from a Middle East trip last month.
All subsequent infections in South Korea have occurred in healthcare facilities and been traced to the original patient.
South Korea has the second highest number of infections, after Saudi Arabia, according to data from the European Centre for Disease Prevention and Control.
The seventh reported MERS death in South Korea was a 68-year-old woman who had an existing heart ailment and had been in the emergency room of a Seoul hospital, where a number of previous confirmed cases had been traced.
South Korea's central bank is now expected to cut interest rates on Thursday in a preemptive move to dampen the economic impact of the MERS outbreak, according to a Reuters poll of analysts released on Tuesday.
The Chinese territory of Macau required masks for people entering local healthcare facilities as a precaution against MERS, and advised residents to avoid travel to South Korea unless absolutely necessary.
Some 2,892 people who may have had contact with MERS patients have been put under quarantine, some in hospitals but most at home. Authorities have said they are using mobile phones to track people who violate quarantine.
South Korea's new cases bring the total of MERS cases globally to 1,244, based on World Health Organization (WHO) data, with at least 446 related deaths.