The case became public this week as China continues to raise pressure on Tokyo after Prime Minister Sanae Takaichi suggested in November that Japan may react militarily if Taiwan were to come under an attack.

Beijing claims the self-ruled island as part of its own territory and has not ruled out seizing it by force.

The news also came as Tokyo Electric Power (TEPCO), which operated the crippled Fukushima atomic plant, moves to restart the world's biggest nuclear plant later this month.

A Nuclear Regulation Authority (NRA) employee has lost a work-issued smartphone, used during disasters such as major earthquakes, an agency official told AFP Thursday on the customary condition of anonymity.

The incident was reported in November to a Japanese body that governs proper handling of personal information, he told AFP.

The device is primarily used for telephone calls and texting, and not for accessing nuclear data at the agency, he added.

The smartphone contained the names and contact details of staff members in the authority's nuclear security division, according to major media, including Kyodo News and the Asahi Shimbun newspaper.

Information on staff in the division is not made public due to the sensitivity of their work, local media said.

The employee is believed to have lost the smartphone at an airport in Shanghai on November 3 while taking items out of carry-on luggage during a security check, Kyodo said, citing unnamed sources.

The person realised the phone was missing three days later but was unable to locate it, Kyodo added.

Remotely locking or erasing the data on the phone was also not possible as it was out of range, according to Kyodo.

The NRA is currently evaluating TEPCO's application to restart the world's biggest nuclear plant, Kashiwazaki-Kariwa.

Japan pulled the plug on nuclear power after a colossal earthquake and tsunami sent three reactors at the Fukushima atomic plant into meltdown in 2011.

However, the resource-poor nation now wants to revive atomic energy to reduce its heavy dependence on fossil fuels, achieve carbon neutrality by 2050 and meet growing energy needs from artificial intelligence.