BEIJING — The authorities in China reported a third death from a mysterious virus and more than 130 new cases over the weekend, including ones found in Beijing and southern China for the first time.

The jump in cases raised questions about how the virus is being transmitted and added to concerns about the spread of the illness ahead of China’s busiest travel season.

The health commission in Wuhan, a central Chinese city where the virus originated last month, said early Monday that 136 new cases had been detected on Saturday and Sunday, including one who died. Nine patients in the city were critically ill.

Two new cases were reported in Beijing, and one in Shenzhen, a booming metropolis in the south, near Hong Kong. That brought the total number of cases in China to around 200, more than double the number reported just a day earlier.

The latest cases come amid growing concern among some experts that the outbreak could be more severe than China’s government has described. The virus already appears to have spread outside China, with three cases reported in Thailand and Japan involving people who had traveled through Wuhan.

And with hundreds of millions of people in China expected to travel for the Lunar New Year holiday, which begins Friday, public health officials are working to stop a major outbreak.

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On Sunday, China’s central government sought to reassure the public that it had the situation under control. In Beijing’s most extensive remarks on the outbreak since it began last month, the National Health Commission said that experts agreed that an epidemic was “still preventable and controllable.”

Still, the commission acknowledged that the source of the virus and its path of transmission were not fully understood.

“The mutation of the virus still needs to be closely monitored,” the statement said.

Of the new patients found in Wuhan over the weekend, 66 were men and 70 women, and their ages ranged from 25 to 89, the health commission reported on Monday. It said that they mostly had symptoms such as fever, coughing and difficulty breathing.

Most people with the infection have contracted it through exposure to animals at a market in Wuhan that sells seafood and live animals, the authorities say. But the health commission in Wuhan said on Sunday that some people who had come down with the virus had no exposure to the market.

That acknowledgment raises the possibility that the virus could be present in other markets in Wuhan, experts said, adding to fears that more people might be at risk.

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“If you cannot find the source and control the source of the virus, you cannot extinguish the fire,” said David Hui, the director of the Stanley Ho Center for Emerging Infectious Diseases at the Chinese University of Hong Kong.

Hui said the risk of the virus spreading from human to human on a broad scale appeared to be low, though he noted that the virus could mutate.

The World Health Organization said on Sunday that while its analysis indicated that limited transmission of the virus is possible, it remained unclear whether it can easily spread from one person to another. The group said it would continue to examine the issue.

“We do not have enough evidence to evaluate the full extent of human-to-human transmission,” its Manila office said.

Some experts have suggested that there are probably far more cases of the illness than the authorities have disclosed. In previous incidents, like the 2003 outbreak of severe acute respiratory syndrome, or SARS, the Chinese government withheld critical information.

One estimate by researchers at Imperial College London suggested on Friday that there could now be as many as 1,700 cases of the new virus.

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The WHO said on Sunday that China could potentially confirm more cases in the coming days and weeks as more people were screened for it.

The new virus has brought back memories of the SARS outbreak, which was also caused by a coronavirus. SARS, which is believed to have jumped to humans from animals at markets, originated in China and spread to other countries, infecting more than 8,000 people and killing more than 800.

While the new coronavirus appears to be less severe than SARS, public health officials around the world are exercising caution.

On Friday, the Centers for Disease Control and Prevention in the United States announced that airports in New York, San Francisco and Los Angeles would begin screening passengers from Wuhanfor the virus.

W. Ian Lipkin, a Columbia University professor who worked to fight the SARS outbreak, said it was too early to know how dangerous the new virus might prove to be.

“Until it becomes capable of human-to-human transmission, there’s not a major threat of a pandemic,” said Lipkin, the director of the Center for Infection and Immunity at the university’s Mailman School of Public Health.

“We need to prepare for the possibility that this could be a larger outbreak, and it could become a pandemic,” he said. “But that doesn’t mean that it will.”