Japanese health authorities are concerned that variants of the coronavirus are driving a nascent fourth wave in the pandemic with just 109 days remaining until the start of the Tokyo Olympics.

The variants appear to be more infectious and may be resistant to vaccines, which are still not widely available in Japan. The situation is worst in Osaka, where infections hit fresh records last week, prompting the regional government to start targeted lockdown measures for one month from Monday.

A mutant COVID-19 variant first discovered in Britain has taken hold in the Osaka region, spreading faster and filling up hospital beds with more serious cases than the original virus, according to Koji Wada, a government adviser on the pandemic.

"The fourth wave is going to be larger," said Wada, a professor at Tokyo's International University of Health and Welfare. "We need to start to discuss how we could utilize these targeted measures for the Tokyo area."

Japan has twice declared a state of emergency that covered most of the country in the past year, most recently just after New Year as the pandemic's third and most deadly wave struck. Officials are now opting for more targeted measures that allow local governments to shorten business hours and impose fines for noncompliance.

Osaka canceled the Olympic Torch relay events there, but Prime Minister Yoshihide Suga has insisted Japan will carry out the Games as scheduled. Suga said Sunday that measures employed in Osaka could be expanded to Tokyo and elsewhere if needed.

There were 249 new infections in Tokyo Monday, still well below the peak of over 2,500 in January. In Osaka, a record 666 cases were reported Saturday.

The true extent of the mutant cases is unknown, as only a small fraction of positive COVID-19 cases undergo the genomic study necessary to find the variants.

A health ministry report last week showed 678 cases of mutant variants from Britain, South Africa and Brazil had been discovered nationwide and at airports, with the biggest clusters in Osaka and nearby Hyogo prefecture.

But another strain, known as E484K, may be more widespread. Around 70% of coronavirus patients tested at a Tokyo hospital last month carried the mutation that was different from the British and South African varieties, Japanese public broadcaster NHK said Sunday.

The rebound in cases came within weeks of the government lifting the state of emergency measures, and the priority measures being rolled out now are intended to halt an unexpected rise in mutant cases, said Makoto Shimoaraiso, a Cabinet Secretariat official for Japan's COVID-19 response.

"We take the criticism (seriously) when people say that we have not been able to detect any variants," he said.