– Major Chinese cities, including the capital and virus-hit Wuhan, banned all large gatherings over the coming Lunar New Year festival, the most important holiday on the Chinese calendar, in an expanding effort to contain the rapidly spreading outbreak. The announcement Thursday came as authorities expanded travel restrictions imposed on Wuhan to surrounding municipalities, shutting down travel networks and attempting to quarantine about 25 million people — more than the population of Florida.

The extreme measures were accompanied by other indications that Communist Party authorities were struggling to control the outbreak, notably the aggressive censorship of any criticism or skepticism on social media. But some outspoken doctors warned that the controls would not be enough to stop the spread of the pneumonia-like virus, which has now killed 25 people in China and infected 835 people.

"A bigger outbreak is certain," said Guan Yi, a virologist who helped identify severe acute respiratory syndrome (SARS) in 2003. He estimated — "conservatively," he said — that this outbreak could be 10 times bigger than the SARS epidemic because that virus was transmitted by only a few "super spreaders" in a more defined part of the country. "We have passed through the 'golden period' for prevention and control," he told Caixin magazine from self-imposed quarantine after visiting Wuhan. "What's more, we've got the holiday traffic rush and a dereliction of duty from certain officials."

The World Health Organization on Thursday cited Chinese efforts to prevent transmission and the limited number of cases recorded abroad as its reasons for not declaring a public health emergency of international concern.

But WHO Director General Tedros Adhanom Ghebreyesus said at a news conference that it "should not be taken as a sign that WHO does not think the outbreak is serious or that we're not taking it seriously. Nothing could be further from the truth."

He said that the emergency committee was divided on whether the outbreak deserved the designation but that the outbreak amounts to "an emergency in China."

"It has not yet become a global health emergency, but it may become one," he said.

In general, declaring an international public health emergency gives the WHO director general powers to issue recommendations to other countries, such as urging them not to close borders or restrict trade with a country in the middle of an outbreak. Public health officials say such measures are considered unlikely to stop disease spread and very likely to discourage countries from being forthright about outbreaks.

Much about how the virus works is unknown, Ghebreyesus added, including its source, how easily it spreads and its full clinical features. These are all critical factors, experts have said, for declaring a global health emergency.

The Lunar New Year holiday, a weeklong period when millions of Chinese travel to their hometowns, officially begins on Friday.