Health experts put Tokyo on the highest alert for coronavirus infections on Wednesday, alarmed by a recent spike in cases to record levels, while the governor of the Japanese capital said the situation was "rather severe".

The resurgence of the virus in Tokyo could add to the growing pressure on policymakers to shore up the world's No.3 economy, which analysts say is set to shrink at its fastest pace in decades this fiscal year due to the pandemic.

"It is a fact that the number of patients is going up quite a bit and exceeding peaks," said Norio Ohmagari, director of the National Center for Global Health and Medicine Hospital, noting that the infection rate in Tokyo was at stage "red" - the highest of four levels in the metropolis' system.

Infections among young people and asymptomatic cases are rising, Ohmagari said at a meeting with Tokyo officials.

Governor Yuriko Koike repeated her calls for Tokyo residents to be vigilant, saying: "My understanding is that we're in a rather severe situation now."

The Tokyo government will respond accordingly to the expert views, Koike said, ahead of a media conference later in the day.

Fearing a second wave of infections spreading from the capital, local municipalities and opposition lawmakers also urged the central government to suspend a major "Go To" travel aid campaign that aims to boost domestic tourism.

Leaders of some rural towns say that driven by the campaign, travel in and out of high-risk regions like Tokyo may lead to widespread community transmissions.

The pandemic in Japan will turn into a "man-made" disaster should the travel programme go ahead, Soichiro Miyashita, mayor of the city of Mutsu in Aomori Prefecture, has cautioned.

But Japan's economy minister, Yasutoshi Nishimura, said the government will cautiously proceed with the campaign, which includes discounts for shopping and food.

"Obviously we will consider the thoughts of many of our people, while monitoring the situation ahead," Nishimura, who leads the government's coronavirus policy, told parliament.

The programme, among the government's top initiatives to stimulate economic activity and set to start this month, has also come under fire over costs as it subcontracts back-office work to a private contractor.

In Tokyo, daily virus cases exceeded 200 in four of the past seven days, touching an all-time high of 243 last Friday as testing among nightclub workers in its red-light districts showed rising infections among people in their 20s and 30s.

Governor Koike reiterated that people should avoid restaurants that lack proper infection control measures.

Health experts noted Tokyo hospitals were getting crowded as the number of patients doubled from the previous week.

Tokyo's latest cluster has been traced to a theatre with at least 37 cases in Shinjuku, a busy entertainment area and home to one of Asia's biggest red-light districts which has been the centre of a recent spike in infections.