TOKYO — Japan will step up border controls at airports after health authorities found 21 cases of a new double-mutant Indian variant experts say could be more contagious.

Chief Cabinet Secretary Katsunobu Kato said Monday all but one of the 21 variant cases were detected in arrivals at Japanese airports.

The variant has two mutations in the spiky protein that the virus uses to fasten itself to cells. India is experiencing the world's worst surge in virus cases partly due to the variant. Experts fear it could worsen Japan's ongoing surge that is being fueled by another variant detected earlier in Britain.

Japan started emergency measures Sunday in Tokyo, Osaka and two of its neighboring prefectures, closing down bars, karaoke, department stores, theaters and museums and other nonessential businesses and requiring staying home and other preventive measures for the residents until May 11.

The third state of emergency in Japan three months before a scheduled opening of the Olympics in July is bringing into question if or how the country can safely hold the events.

Kato said Japan will step up border controls and collect and analyze data on the variants to prevent the further spread of the infections.

Japan had 562,141 cases, up more than 5,400 from the day before, and 9,913 deaths on Friday, according to the health ministry.