Japan is likely to extend by two weeks or more coronavirus containment measures in the greater Tokyo area, four government sources said on Thursday, as infection numbers creep up less than a month before the Summer Olympics start.

Japan's capital and three neighbouring prefectures are among areas under a 'quasi' state of emergency set to run through 11 July, but a recent uptick in infections has officials leaning towards keeping restrictions in place - a move that could affect the number of spectators allowed into Olympic venues, the sources said.

Depending on the extent of the strain on the medical system, the government could reinstate a full state of emergency for Tokyo, the sources said. New infections in the Olympics' host city rose to 714 on Wednesday, the highest in more than a month.

A decision is expected around 8 July, when International Olympic Committee President Thomas Bach is due to arrive in Japan, the sources said.

The Olympics are set to start on 23 July but many in Japan remain opposed to holding the event, which medical experts have warned could unleash another wave of infections.

The 'quasi' state of emergency caps spectators at 5,000. Olympics organisers have said spectators will be allowed up to half of venue capacity or a maximum of 10,000 provided the emergency restrictions are lifted.

Spectators from overseas have already been banned, and some members of the ruling coalition are beginning to favour having no spectators at the Olympics, the sources said.

Samoa has become the second country to withdraw some athletes from the Games as fears grow of a depleted event being staged under a renewed state of emergency.

Meanwhile, visiting officials and media have been handed a Big Brother-style warning that that they risk being watched and shamed on social media if they fail to adhere to stringent quarantine conditions.

The Samoan Olympic Committee confirmed it has withdrawn three weightlifters who had qualified for Tokyo, but said eight other team members, none of whom are based on the Pacific island, will still attend the Games.

A statement said the decision over the weightlifters, who are all based in Samoa, was taken reluctantly in order to "protect our borders", citing the continued struggle to contain the virus on nearby Fiji.