Japanese Prime Minister Fumio Kishida has said his country faces the severest security environment in the region since the end of the Second World War as he outlined his government’s priorities.

In a speech opening this year’s parliamentary session, he pledged to push a military build-up under a new security strategy over the next five years and beyond, as well as tackling rapidly declining births so the country can sustain national strength.

Mr Kishida’s government adopted key security and defence reforms in December, including a counter-strike capability that marks a departure from the country’s exclusively self-defence-only post-war principle.

Japanese officials say the current deployment of missile interceptors is insufficient to defend it from rapid weapons advancement in China and North Korea.

In his policy speech, Mr Kishida said active diplomacy should be prioritised, but stressed that it requires “defence power to back it up”.

He said Japan’s new security strategy is based on a realistic simulation “as we face the most severe and complex security environment since the end of World War Two, and a question if we can protect the people’s lives in an emergency”.

The strategy seeks to keep in check China’s increasingly assertive territorial ambitions, but it is also a sensitive issue for many countries in Asia that were victims of Japanese wartime aggression.

Mr Kishida said it is a “drastic turnaround” of Japan’s security policy, but still remains within the limitations of its pacifist constitution and international law.

He said: “I make it clear that there will not be even a slightest change from Japan’s non-nuclear and self-defence-only principles and our footsteps as a peace-loving country.”

This month, Mr Kishida took a five-nation tour, including Washington, to explain Japan’s new defence plan and further develop defence ties with its ally the United States.

Japan plans to nearly double its defence budget within five years to 43 trillion yen (£267 billion) and improve cyberspace and intelligence capabilities.

While three-quarters of an annual defence budget increase can be squeezed out through spending and fiscal reforms, the remainder needs to come from a possible tax increase, and Mr Kishida has already faced growing criticism from opposition legislators and his governing party.

Mr Kishida also faces a critical question of population growth.

“We cannot waste any time on the policies for children and child-rearing support,” he said. “We must establish a children-first economic society and turn around the birth rate.”

Japan’s population of more than 125 million has been declining for 14 years and is projected to fall to 86.7 million by 2060.

A shrinking and ageing population has huge implications for the economy and national security.

Mr Kishida pledged to bolster financial support for families with children, including more scholarships, and said he would compile a plan by June.

Japan is the world’s third biggest economy, but living costs are high and wage increases have been slow. The conservative government has lagged behind on making society more inclusive for children, women and minorities.

So far, efforts to encourage people to have more babies have had limited impact despite payments of subsidies for pregnancy, childbirth and child care.

Some experts say government subsidies still tend to target parents who already have children rather than removing difficulties that are discouraging young people from having families.