Japan's Finance Minister Taro Aso has reluctantly apologised for saying childless people are to blame for the country's rising social security costs and its ageing and declining population.

“If it made some people feel uncomfortable, I apologise,” Aso said on Tuesday after drawing complaints over a comment he made during the weekend at a seminar in Fukuoka, his constituency in southwestern Japan.

The gaffe-prone Aso, a 78-year-old former prime minister, is among conservative lawmakers in Prime Minister Shinzo Abe's government who have at times blamed the elderly or childless for long-term demographic trends.

“There are lots of strange people who say the elderly people are to blame, but that is wrong. The problem is those who don't have children,” he told the audience.

The comment was nearly the same as one he made in 2014 that also drew criticism.

According to the latest government statistics, the number of births in 2018 fell to 921,000, the lowest since Japan began recording such statistics in 1899. Japan's total population fell by 448,000 people, a record decline, to 126 million.

It is forecast to fall below 100 million by 2050, barring a huge influx of immigrants.

Abe himself has no children. He has acknowledged that lack of access to affordable child care, excessively long working hours, elder care and other realities of life, especially in Japan's biggest cities, are major factors behind the country's low birthrate.

But promised labour and other reforms to help alleviate the burden on families that discourage couples from having more children have made limited headway.

Japan is known for longevity, another factor behind the ageing of its population and rising costs for elder care.

Aso said his comment was taken out of context and misunderstood, but opposition lawmakers disagreed.

“He not only lacked consideration to those who choose not to or cannot have children, but he just doesn't understand what the problem is,” said opposition lawmaker Kiyomi Tsujimoto, who belongs to the Constitutional Democratic Party of Japan, on Monday.

“He has no sense of human rights.”