A Sars-like virus that has spread across China and reached three other Asian nations is contagious between humans, a government expert said on Monday.
Meanwhile, the World Health Organisation announced that an emergency committee would meet this week to discuss the infections.
The new coronavirus strain, first discovered in the central city of Wuhan, has caused alarm because of its connection to Severe Acute Respiratory Syndrome, which killed almost 650 people across mainland China and Hong Kong in 2002 and 2003.
The number of diagnoses of the new virus has risen to 218.
Beijing and Shanghai confirmed their first cases on Monday while more than a dozen more emerged in southern Guangdong province and 136 were found at the weekend in Wuhan, state broadcaster CCTV reported.
A third person died in Wuhan, the local health commission said.
Scientists are desperately trying to determine the mode of transmission, with a seafood market in Wuhan believed to be the centre of the outbreak.
But Zhong Nanshan, a scientist at the National Health Commission who helped to expose the scale of the Sars outbreak, said patients could contract the new virus without having visited the city.
Dr Zhong told CCTV the new strain had been transmitted between humans.
In Guangdong, two patients were infected by family members who visited Wuhan, he said.
Fourteen medical personnel helping with coronavirus patients were also infected, he said, although more than 95 of the cases were related to Wuhan.
Dr Zhong predicted an increase in viral pneumonia during the Lunar New Year holiday, when millions travel in China, but was confident the spread of the virus would be curbed, China's official Xinhua news agency reported.
The WHO panel will meet in Geneva on Wednesday to determine whether to declare the outbreak "a public health emergency of international concern", which is only used for the most serious epidemics.
The WHO said earlier that an animal source seemed to be the most likely primary source, with "some limited human-to-human transmission occurring between close contacts".
Wuhan has a population of 11 million and is a major transport centre during the annual Lunar New Year holiday that begins this week, when hundreds of millions of Chinese travel across the country to visit family.
On Monday, President Xi Jinping said protecting people's lives would be given top priority and that the spread of the epidemic "should be resolutely contained".
Mr Xi said it was necessary to "release information on the epidemic in a timely manner and deepen international co-operation", and ensure people have a "stable and peaceful Spring Festival".
Five cases were reported in Beijing, while in Shanghai a woman, 56, who had travelled from Wuhan, was admitted to hospital and was in stable condition, health authorities said.
South Korea on Monday also reported its first case. The woman, 35, flew in from Wuhan.
Thailand and Japan have previously confirmed a total of three cases, all of whom had visited the Chinese city.
There are also six suspected cases in Shanghai and in four provinces and regions in the east, south and south-west of the country.
The virus did not slow down the annual holiday rush, although some travellers wore masks at crowded railway stations in Beijing and Shanghai.
"Watching the news, I do feel a little worried," said Li Yang, 28, an accounts manager who was heading home to Inner Mongolia for the Lunar New Year.
"But I haven't taken precautionary measures beyond wearing regular masks."
The WHO said the new cases in China were the result of "increased searching and testing among people sick with respiratory illness".
Scientists with the MRC Centre for Global Infectious Disease Analysis at Imperial College in London warned in a paper published on Friday that the number of cases in Wuhan was likely to be close to 1,700.
Wuhan authorities said they installed infra-red thermometers at airports and railway and coach stations across the city. Passengers with fever were registered, given masks and taken to medical centres.
State TV showed medical staff working in hazardous material suits in an isolation ward at a Wuhan hospital.
Hong Kong health officials said they were expanding enhanced checks on arrivals to include anyone travelling from Hubei province, not just its capital Wuhan. More than 100 people are being monitored in the city.
Passengers are also being screened at some airports in Thailand and the US.
In Wuhan, 170 people were still being treated at hospital, including nine in critical condition, the health commission said.