Japanese Prime Minister Shinzo Abe pledged on Friday to bolster his country's defences, calling the threat from North Korea the gravest security concern Japan has faced since World War II.

Outlining his priorities in a policy speech to parliament, Abe described North Korea's sixth nuclear test earlier this year and two missile launches that flew over Japan as a national crisis.

He promised concrete action to respond to what he called “escalating provocations” by North Korea.

“We will strengthen Japanese defence power, including missile defence capabilities, in order to protect the people's lives and peace,” he said.

Abe called on the international community to put more pressure on North Korea to persuade it to change its policies.

The 39-day special parliamentary session runs through December 9. Abe's ruling coalition won in a landslide in last month's snap election, securing a two-thirds supermajority in both houses of parliament, which makes it easy for Abe's policies to be approved.

Japan's defence spending has increased slowly but steadily since Abe took office in 2012. The government has said it plans to buy more American missile defence systems.

During his visit last week in Tokyo, President Donald Trump urged Abe to buy many more American weapons that allow Japan to shoot down North Korean missiles, raising questions if he was more interested in the business side of it.

US Ambassador to Japan William Haggerty stressed that Trump's primary focus is on security, not trade. He said the US is trying to make more advanced weapons technology available to Japan more efficiently and US defence more effective in the region.

“Our overarching goal is to increase Japan's capability and interoperability. Our overarching goal is on security and defence,” Haggerty said. “And (Japan's) goal is to make certain that more advanced technology is available to Japan.”

He said there may be a positive impact on the trade deficit as well “but I assure you that's not the primary reason for this program.”