Prime Minister Fumio Kishida's son has been given a strong warning over inappropriate photographs taken at the prime minister's residence during a family function last year, the top government spokesman said Thursday.

The caution given to Shotaro Kishida, who works as an executive secretary to his father, was disclosed by Chief Cabinet Secretary Hirokazu Matsuno after a weekly magazine reported that he posed for photographs with relatives at a staircase and other symbolically important parts of the residence in a manner seen as disrespectful.

Speaking at a regular press conference Thursday, Matsuno said the government is aware that the actions of the prime minister's eldest son "lacked appropriateness."

Japanese Prime Minister Fumio Kishida's eldest son Shotaro. (Kyodo)

"We will ensure proper use and management (of the facility) to ensure such an incident does not occur again," he added.

According to reporting by the weekly magazine Shukan Bunshun, Shotaro Kishida and around 10 others, including relatives, posed for a photo last December on the red-carpeted staircase, where Japanese prime ministers traditionally debut their new Cabinet lineup with a photo call.

The prime minister last did so in August following a reshuffle. One of the photographs in question features his son and others dressed casually while posing as ministers do.

Among the other photos released by the magazine are two of a man and woman, who appear to be relatives of the Kishida family, posing at the prime minister's lectern, which is typically used for press conferences.

Matsuno said the photos were taken during a year-end party for relatives, in which the prime minister made an appearance.

While the prime minister's residence accommodates the leader and his family, it is a public building for carrying out official business and hosting functions on behalf of the state, Matsuno said.

Regarding Shotaro Kishida's conduct, Matsuno said he hopes he will "carry himself with a greater sense of care when carrying out his duties."

Later Thursday, the premier told reporters that he has no intention of sacking Shotaro Kishida as an executive secretary, although he said he "deeply regrets" his son's behavior as it lacked "appropriateness."

Kishida's eldest son has proved a repeated source of trouble for the prime minister since he was tapped for the executive secretary role in October last year.

The prime minister has been criticized by opposition politicians for nepotism over the appointment. Matsuno rejected the accusations at the time, saying that the prime minister's eldest son was offered the job based on his "personality and insight."

In January, the younger Kishida found himself under fire from opposition politicians, who said he inappropriately mixed public and private interests by going sightseeing while accompanying his father on visits to other Group of Seven advanced nations ahead of a Hiroshima summit, held in May.

News of the incident initially came from another weekly magazine. As part of what the prime minister later called "official duties," the younger Kishida was reported to have used a government car to visit major tourist spots and go shopping in London, Paris and Ottawa.

The government said there was no problem with his use of the vehicle because he took photos for his father's social media and purchased gifts on the senior Kishida's behalf.