Tokyo's coronavirus outbreak is severe and could explode in the coming days just as Japan begins its New Year's holiday period, in which millions of people usually move around the country, the city's governor said on Wednesday.

The capital recorded 944 new coronavirus cases on Wednesday, just under the record 949 recorded on Saturday, and medical experts warned that unless the outbreak is checked the city could soon see over 1,000 new patients a day.

"Please emphasize life over fun," Gov Yuriko Koike told a news conference, calling on people to stay at home as much as possible over the holiday.

Tokyo was at a critical juncture, with cold winter weather forecast and highly contagious variant strains of the coronavirus from Britain and South Africa already detected in Japan, Koike said.

"The situation is extremely severe, we are facing up to a huge wave of virus cases we have not seen before," she added. "The number of cases could explode at any time. Any one of us could become infected at any time ... If we do not stamp this out now, we will be facing a much worse situation."

The Japanese government on Monday started banning the entry of non-resident foreign nationals following the discovery of the UK variant in Japan, and has been calling on citizens to refrain from the parties, family gatherings and travel that are usual at this time of year.