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.