"Osaka particularly is in quite a dangerous situation with its medical system," Nishimura said on Friday at the start of a meeting with a panel of medical and economic experts, noting that variant strains are spreading rapidly.
"We have a strong sense of danger that Tokyo could soon be turning into the same situation as Osaka."
The panel later approved the extension plan, Nishimura said. Formal government approval is set for later in the day, with Prime Minister Yoshihide Suga giving a news conference at 7 p.m. (1000 GMT).
All hospital beds for critical patients in Osaka are occupied, public broadcaster NHK reported on Friday.
Nationwide, Japan has recorded 618,197 cases of novel coronavirus infection and about 10,585 deaths from COVID-19, the respiratory disease the virus causes, government figures showed.
Nishimura said the government aims to place Aichi prefecture to the west of Tokyo and Fukuoka prefecture in the southwest under a state of emergency - joining Tokyo and Osaka, Hyogo and Kyoto prefectures, home to nearly a quarter of the population.
The northern island of Hokkaido and two other areas will be added to regions under a "quasi state of emergency," he said.
Under the extended state of emergency, bars, restaurants, karaoke parlours and other places serving alcohol will continue to be asked to stop serving alcohol, while people will continue to be asked to avoid taking unnecessary trips.
But other requests will be loosened, Nishimura said. Commercial facilities such as shopping malls, for instance, will be asked to shorten opening hours rather than close completely - though he noted that Tokyo and Osaka would make their own decisions based on local conditions.
Japan has not suffered as badly from the virus as other nations, but its vaccination campaign is significantly lagging, with even many elderly people still awaiting inoculation.
Still, Japan and the International Olympic Committee insist the Games will take place, though foreign spectators have been banned. A decision on domestic spectators has yet to be reached.
Upcoming test events for the Olympics, including an athletics event at the weekend, will not be affected by the extension of the state of emergency. The diving World Cup, which featured more than 200 athletes from 50 countries, was held in Tokyo this past week under the current state of emergency.
U.S. pharmaceutical maker Pfizer Inc and German partner BioNTech on Thursday said they would donate doses of their novel coronavirus vaccine to help inoculate athletes and other members of Olympic delegations.
Still, in Fukuoka, the prefectural governor said it could be "difficult" to host the Olympic torch relay under a state of emergency, Jiji news agency reported on Thursday. The relay is set to take place in the prefecture on May 11 and 12.
(Reporting by Elaine Lies and Ritsuko Ando; Editing by Richard Pullin and Christopher Cushing) ((email@example.com; +81-3-4563-2748; Reuters Messaging: firstname.lastname@example.org))