China’s Communist Party leadership called the month-old coronavirus epidemic a “major test” on Monday as other nations escalated efforts to isolate China, unnerving China’s stock market, depressing global oil prices and raising new anxiety about the world’s most populous country.

The growing global move to effectively cut off China’s 1.4 billion people came as government officials reported the new coronavirus strain had killed more in mainland China, 425 as of Tuesday morning, than the SARS outbreak in 2002 and 2003, confirming it as one of the deadliest epidemics in recent Chinese history.

Many leading infectious disease experts say the outbreak is likely to become a pandemic, defined as an ongoing epidemic on two or more continents, and that stringent anti-contagion restrictions may have come too late.

“There’s no sign that it’s getting better,” said Leo Poon, division head of the public health laboratory sciences department at the University of Hong Kong. “We don’t see a pattern of decline, and that’s a problem.”

Mr. Xi presided over a meeting of senior Communist Party leaders at which they acknowledged shortcomings in policies on public health and emergency management, according to a report by China’s official news agency. The leaders called the coronavirus epidemic “a major test of China’s system and capacity for governance.”

Xinhua quoted Mr. Xi as saying that officials who resist orders and “lack boldness” could be punished — suggesting that at least some regions in China may have balked at devoting resources and personnel to stopping the contagion.

As of Monday, China had 20,438 cases, the government said on Tuesday morning, and more than 160 cases have been diagnosed in two dozen other countries, including 11 in the United States. During the SARS outbreak, China had 349 deaths and 5,327 cases, according to the World Health Organization.

The commonly accepted need for isolating suspected cases collided with anger, confusion and recrimination between China and other governments.

In the United States, there were scenes of uncertainty at the few airports still permitted to receive flights from China, as the first federally required quarantine since the smallpox era a half century ago took effect.

Russia, which shares a 2,600-mile border with China, suspended all passenger-rail links. In the Philippines, President Rodrigo Duterte exhorted citizens to “stop this xenophobia thing” amid signs there were acts of discrimination against people of Chinese descent.

The government of Hong Kong, the semi-autonomous territory that is part of China, closed four border crossings to the Chinese mainland, leaving just three, as more than 2,400 Hong Kong medical workers went on strike to press for a total ban on mainland arrivals.

A spokeswoman for the Chinese Foreign Ministry, Hua Chunying, criticized the American response, adding that, “the U.S. government has not provided any substantive help to the Chinese side yet.”

In an online news briefing, Ms. Hua noted that the United States was “the first to withdraw its consulate staff from Wuhan, the first to suggest the partial withdrawal of embassy staff and the first to announce a ban on entry by Chinese citizens.”

“What the U.S. has done could create and spread panic,” Ms. Hua said.

But in China itself, millions of people who were working in Hubei Province have been stopped from returning to their home areas, feared as potential carriers of the disease and treated as outcasts. Even those without symptoms are being ostracized.

Last week, the American health and human services secretary, Alex Azar, said that he had offered to send a team from the Centers for Disease Control and Prevention to China to help with the coronavirus outbreak, adding that he had reiterated the offer several times.

Even as Chinese officials tried to reassure their own public that shortages of medical supplies were being addressed, and that food prices were stable, the spillover effects of China’s isolation reverberated through the Chinese stock market, which had been closed since Jan. 23 for the Lunar New Year holiday. Investors confronting the prospect that the world’s No. 2 economy could suffer severe constraints sent stock prices tumbling by 8 percent.

In a note to clients, Tai Hui, J.P. Morgan’s chief market strategist in Asia, wrote that, “As the number of infections is still likely to rise in the weeks ahead, we would expect the Chinese onshore equity market to come under pressure.”

The anxiety also infected global energy markets, where the possibility of falling demand from a hobbled China — the world’s biggest importer of oil — sent prices to the lowest level in more than a year. Ministers from the Organization of Petroleum Exporting Countries, as well as Russia, agreed to meet on Tuesday and Wednesday about possible production cuts.

In Wuhan, ailing residents have been begging for beds at local hospitals. Overwhelmed doctors have run out of medical supplies. In response, the Wuhan government announced that two new hospitals were to be built within weeks. The first hospital, with 1,000 beds, opened Monday after it was built in just eight days.

The Chinese National Health Commission reported on Sunday that there had been 475 recoveries. In Hubei, 80 patients were pronounced recovered on Sunday, while 56 died that day. On Saturday, 49 patients were released from hospitals, and 45 died.

China has sealed off several of its cities, including Wuhan, restricted public gatherings and quarantined some communities. Many cities have been brought to a virtual standstill as residents have been told to stay at home and schools and offices remain shut.

Although many Chinese cities have extended the Lunar New Year holiday to combat the spread of the disease, public health experts say the virus is still likely to spread, given how infectious it is and the large number of travelers expected to commute for work.

The geographical extent of the disease is reminiscent of that of SARS, with cases reported in at least 25 countries, amplifying fears that the virus could spread across the world.