TOKYO (REUTERS) - Japan's serious Covid-19 cases crossed 1,000 for the first time in four months, data showed on Friday (Feb 4), as the Omicron variant fuelled record infections and burdened the medical system.

The number of seriously ill patients climbed by 131 to 1,042 cases from the day before, the Health Ministry said, the highest since September, when the Delta variant drove a fifth wave of cases.

Japan recorded 96,748 new cases on Thursday.

Most regions are now under infection control measures to try to blunt the spread of Omicron that has exploded among a population where less than 5 per cent have received vaccine booster shots.

The government is considering a two-week extension of the curbs in 13 regions, including its capital Tokyo, the Fuji News Network said on Thursday.

Tokyo raised its coronavirus alert to the highest level on Thursday. The city government also laid out revised criteria for requesting a full state of emergency.

Tokyo will request an emergency declaration if either the occupancy rate of hospital beds for serious patients or the rate of patients needing oxygen reaches 30-40 per cent, and the seven-day average of new cases hits 24,000.