Japanese officials are urging users of disrupted KDDI mobile phone networks to use other carriers or fixed-line phones to make an emergency call, or go to the nearest fire station.

The Fire and Disaster Management Agency issued the notice on Saturday after major disruptions occurred to KDDI's phone and data communication services across Japan.

The Ministry of Internal Affairs and Communications asked KDDI to restore its services as quickly as possible and make sure its users are fully informed.

The ministry says that it is a serious concern that people cannot use their mobile phones when a tropical storm is approaching Okinawa and they could suffer from a natural disaster.

Mobile phone carriers are required by law to report to the communications ministry about a "severe accident", defined as an incident which affects more than 30,000 subscribers for at least one hour and disrupts emergency call services.

Last October, major disruptions occurred on the 3G, 4G and 5G networks of another Japanese mobile carrier NTT Docomo. It affected 4G users, the largest proportion of NTT Docomo subscribers, for more than 10 hours.

It took the carrier 29 hours to get its 3G communications fully operational again.

The communications ministry found the outage to constitute a severe accident as it affected about 1 million users and issued administrative instructions to the company.