The U.S. Embassy in Tokyo issued a warning to American citizens Friday that said those with plans to return should do so now or risk being stuck here for an “indefinite period.”

Singling out Japan’s lack of widespread testing, the embassy gave a sobering assessment of the potential strain the novel coronavirus could place on Japan’s health care system should infections spike.

“The Japanese Government’s decision to not test broadly makes it difficult to accurately assess the COVID-19 prevalence rate,” the embassy said on its website, referring to the illness caused by the virus.

“While we have confidence in Japan’s health care system today, we believe a significant increase in COVID-19 cases makes it difficult to predict how the system will be functioning in the coming weeks,” it said.

If U.S. citizens want to return to the United States from Japan they should do so now or risk staying abroad for an “indefinite period,” it said. It also noted that Americans with pre-existing medical conditions may not be able to get the medical care they were used to receiving before the COVID-19 pandemic struck.

It also noted that only 11 percent of airlines’ pre-pandemic flight capacity from Japan to the United States remained in operation as of Friday. Since Japan’s entry restrictions will further reduce availability in the near future, getting back for a family emergency in a timely manner could become difficult or even impossible, the embassy said.

It thus urged U.S. citizens to work with their airlines to make travel arrangements while limited commercial flights are still available, noting that travelers seeking return fights should do so immediately in light of the potential for rescheduling or cancellations.

Despite the spread of the contagion, the Japanese government has so far been reluctant to pull the lockdown trigger, warning of the heavy damage that doing so might inflict on the world’s third-biggest economy, which is already closing in on yet another recession.

Instead, Prime Minister Shinzo Abe has urged school closures and called on citizens to avoid unnecessary and non-urgent gatherings and outings while preparing to roll out an economic support plan next week, even as he acknowledged the country was barely avoiding a major jump in infections.

According to official data, Japan has so far been spared the kind of explosive surge seen in parts of Europe, the United States and elsewhere, with nearly 3,300 cases and 78 deaths so far. Globally, coronavirus cases surpassed 1 million on Thursday, while deaths have topped 50,000.

The comments from the embassy came after Tokyo Gov. Yuriko Koike said that declaring a national state of coronavirus emergency would send a “strong message” that could help prevent a bigger outbreak, her most explicit nudge so far to the government.

That would give governors legal authority to ask people to stay home and for businesses to close, but not to impose the kind of lockdowns seen in other countries. In most cases, there are no penalties for ignoring requests, although public compliance would likely increase with an emergency declaration.

Nobuhiko Okabe, director-general of the Kawasaki City Institute for Public Health, said judging the timing for an emergency declaration is tough: Do it too soon and it has a big economic impact and serious effect on society; too late and the infections rise.

Tokyo has reported the most infections in the country with nearly 800, a tiny amount compared with its core population of nearly 14 million. But experts are worried about the rise in cases that can’t be traced. As of the end of March, the health ministry had counted 26 clusters nationwide.

Asked what further steps would be taken in the capital if the central government declares a state of emergency, Koike said a lockdown would entail asking residents to refrain from holding events and a repeat of the same kinds of pleas she had already made.

These include working from home where possible and avoiding bars and nightclubs — advice many have yet to heed, although coffee chain Starbucks and clothing retailer Uniqlo joined a string of other businesses in saying they would shutter dozens of stores this weekend.

The government on Friday told the hardest-hit regions to save hospital beds for severely ill patients, while keeping others with milder or no symptoms at home or in hotels. Until now Japan has been hospitalizing all coronavirus patients, regardless of whether they are asymptomatic or have mild symptoms.

But beds are filling up in Tokyo and threaten to elsewhere, experts said this week.

Koike said Tokyo, for its part, would send people with light or no symptoms — the majority of the 628 hospitalized with the coronavirus as of Thursday — home or to hotels.

The capital was working with the government to secure accommodations, she said, while Prime Minister Abe has said utilizing facilities that had been set up for the Olympics — now postponed by a year — was under consideration.

RELATED PHOTOS A man with a mask sweeps the entrance to a building in Tokyo Thursday. | AP