BEIJING — As new cases of the coronavirus spiked on two continents, the World Health Organization warned on Monday that the world was not ready for a major outbreak, even as it praised China’s aggressive efforts to wrest the epidemic under control.

After two weeks on the ground in China, a team sent by the W.H.O. concluded that the draconian measures China imposed a month ago may have saved hundreds of thousands of people from infection. Such measures — sealing off cities, shutting down businesses and schools, ordering people to remain indoors — have provoked anger in China and could be difficult to replicate in democratic countries with a greater emphasis on protecting civil liberties.

“There’s no question that China’s bold approach to the rapid spread of this new respiratory pathogen has changed the course of what was a rapidly escalating and continues to be a deadly epidemic,” said Bruce Aylward, a Canadian doctor and epidemiologist who has overseen international campaigns to fight Ebola and polio and who led the W.H.O. delegation.

The epidemic has already killed more than 2,600 people in China, mostly in Hubei Province, where the outbreak began in December, and infected more than 77,000 people. But the number of new infections in China has been steadily dropping, giving officials in the country confidence that the extraordinary measures have been effective in blunting the virus’s spread.

There are concerns, however, that as people begin returning to work in China, the virus could flare up again.

At the same time, new cases are escalating outside China. In Italy, where there has been an eruption of more than 150 cases, the authorities have locked down at least 10 towns, closed schools in major cities and canceled sporting events — all moves that are echoes of China’s tactics, if not quite as draconian.

In Iran, the outbreak has killed at least 12 people as of Monday, the largest number of coronavirus-linked deaths outside China.

South Korea on Monday reported 231 additional cases, bringing the nation’s total to 833 cases and seven deaths. By Tuesday, another 60 infections had been recorded, bringing the total to just under 900.

Dr. Aylward said responding swiftly and aggressively to contain outbreaks and treat those infected was paramount.

“We have all got to look at our systems because none of them work fast enough,” Dr. Aylward said.

The virus that has crippled China for more than a month now threatens to become a pandemic that could touch virtually every part of the globe. Stock markets in Asia, Europe and North America plunged on Monday as investors worried that the economic disruption the outbreak has already caused in China is all but certain to have wider impact.

The S & P 500 dropped nearly 3 percent in early trading on Monday, after European markets recorded their worst day since 2016, and major benchmarks in Asia closed sharply lower. The Dow Jones industrial average fell more than 900 points in the first hours of trading.

China, which was the source of the outbreak, might also offer solutions, according to Chinese officials and the W.H.O.’s assessment, despite the confusion and obfuscation that slowed the government’s initial efforts to respond to what was then a mysterious new illness appearing in hospitals in Wuhan, the epicenter, in December.

Since late January, the Chinese government has put at least 760 million people — more than half of its population — under residential lockdowns of varying strictness, from checkpoints at building entrances to hard limits on going outdoors, according to a New York Times analysis of government announcements in provinces and major cities.

While China’s reporting has been at times confused — with changes to its method of counting causing huge swings in daily tolls — the overall trend since the middle of this month has indicated a slowing in the rate of infections.

On Sunday, 24 Chinese provinces reported no new cases. Six of them lowered their emergency response measures. In Hubei Province there were 398 new cases, the second consecutive day in which the number of new cases declined.

“The decline we are seeing is real,” Dr. Aylward said.

Even so, the death toll continues to rise, with 150 deaths reported on Sunday, the highest in nearly three weeks. In total, 2,663 people in China have been killed by the virus, according to official figures released Tuesday morning.

Liang Wannian, a senior official with China’s National Health Commission, said China was not ready to declare victory yet.

“The situation is still very grim,” he said at a news conference. “We haven’t stopped the epidemic in Wuhan yet.”

Many health experts agree it is premature to celebrate given the highly contagious nature of the virus and the potential for a new surge in cases when millions of people go back to work in China or when travel restrictions are lifted.

But they generally agreed with the W.H.O.’s assessment on China’s measures.

“The containment definitely worked in China,” said Leo Poon, the head of the public health laboratory sciences division at the University of Hong Kong. “The question now is whether similar policies can be applied in other countries.”

Clarence Tam, an assistant professor of infectious diseases at the School of Public Health at the National University of Singapore, said it was difficult to interpret the case numbers from China, particularly from Hubei. That is because the total number of infections jumped when the authorities expanded the methods used to diagnose them twice in two weeks.

“Trying to look at the case numbers is very difficult,” Dr. Tam said. “We don’t really know what is influencing those case numbers.’’

Adding to the confusion, Chinese media outlets reported on Monday that Wuhan would begin easing a sweeping lockdown, by allowing some people to leave. But just hours after news of the change emerged, the authorities backtracked, saying the announcement had been made in error.

What is unclear to many public health experts is whether a shortage of testing kits is causing a large number of cases to remain undetected. Hospitals in China remain overstretched and many patients say they have been turned away. Health care workers are still coming down with the virus despite official pledges to protect them. Mr. Liang, the health official, said more than 3,000 health care workers have been infected.

Another problem is that China does not disclose how many people are being tested. If the proportion of people being tested is really declining, it would suggest there is a downturn in the rate of transmission. “But we don’t have that yet,” Dr. Tam said.

“From my perspective, it’s ‘watch and wait and see,’ ” he said. “It looks positive but it’s difficult to interpret what those numbers mean at the moment.”

In a speech on Sunday, China’s leader, Xi Jinping, called the epidemic the country’s most serious public health crisis and said it was “the most difficult to prevent and control” since the founding of the People’s Republic.

The epidemic has already severely disrupted life and commerce — as well as the Communist Party’s annual legislative conferences that had been scheduled to begin in Beijing in early March. The Standing Committee of the National People’s Congress announced on Monday that it had postponed the conferences indefinitely.

Mr. Xi said controlling the outbreak in Wuhan and Hubei as well as preventing the epidemic from spreading to Beijing, the capital, were the country’s top two strategic goals. He pledged more pro-growth policies to help overcome the epidemic.

David Heymann, an epidemiologist at the London School of Hygiene and Tropical Medicine, said the case numbers from China suggest that there “may be a decrease in transmission.”

China was following its playbook from the severe acute respiratory syndrome outbreak of 2002-2003, said Dr. Heymann, a former chief of communicable diseases at the W.H.O., when it was “able to stop outbreaks outside the epicenter in Guangdong Province by meticulous outbreak containment and control.”

The real test could be yet to come. As China moves to restart its economy, the coronavirus could flare up again.

“There is an acute recognition here that just as we — the Chinese — forced the tail of this outbreak down, it could come back up again as people start to move again, the shops start to open, the restaurants open, the schools open,” Dr. Aylward said. “It’s a risk.”

Steven Lee Myers reported from Beijing and Sui-Lee Wee from Singapore. Amber Wang and Claire Fu contributed research in Beijing.