Tokyo — The U.S. Embassy in Tokyo has warned Americans of a "significant increase" in the number of coronavirus infections in Japan, and urged them to leave the country now unless they plan to stay indefinitely.

"If U.S. citizens wish to return to the United States, they should make arrangements to do so now," the embassy said in a notice posted to its website, "unless they are prepared to remain abroad for an indefinite period."

People wearing face masks wait to cross a road on April 3, 2020 in Tokyo, Japan. Although Japan has so far been spared the explosive surge of COVID-19 infections seen in some European countries, Prime Minister Shinzo Abe is coming under increased pressure to take tougher measures to tackle the rising infection rate. Getty

The U.S. State Department's current advisory for Japan stands at "Level 2," which calls on Americans to "exercise increased caution." A global health advisory says Americans should "avoid all international travel."

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Japan reported more than 650 new COVID-19 infections over the past three days, with the country's biggest jump happening April 2.

With anxiety rising that a flood of new cases could be imminent, restaurants and shops are nearly empty, traffic on roads and public transport has plunged, and more people are teleworking.

An explosive surge in infections is inevitable if Japan doesn't rapidly adopt tough restrictions like those in the U.S. and Europe, Hokkaido University Professor Hiroshi Nishiura has told Nikkei.

"Voluntary stay-home guidelines so far have only cut person-to-person contact on public transit by 20%, but contact needs to be cut by at least 80%," the forecasting expert said. Unless control measures are stepped-up, his modelling predicts new cases in Tokyo will peak at 6,000 per day.

Over the past three days, infections have averaged about 200 per day — enough to make most of Japan's residents take warnings more seriously.

Tokyo Governor Yuriko Koike has called on the capital's 14 million residents to stay home this weekend, as she did last weekend. She lacks the power to order a mandatory, enforceable lockdown.

Japanese Prime Minister Shinzo Abe would need to declare a national emergency to give the governors of Japan's 47 prefectures the power to implement mandatory lockdowns, if deemed necessary.